Hi All,


As above really. What I need to do is either create, or make an existing, IDictionary<TKey, TValue> read-only. I notice that Dictionary<TKey, TValue> implements the IsReadOnly property from the System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>> interface but I can't find any way that I could make a Dictionary<TKey, TValue> that would return true for this property and complain if you tried to modify it.

I guess what I could do is subclass Dictionary<TKey, TValue> but I'm not sure that's workable since I don't think some of the key members are actually virtual, which leaves creating a class called ReadOnlyDictionary<TKey, TValue> which would simply wrap another IDictionary<TKey, TValue> implementation... unless of course there's something simpler I can do?

Anyone got any ideas?


Many thanks,
Bart

---

Generation X.1

Hi Bart,

I would create a wrapper around Dictionary<> and implement readonly property. And you are correct, you can't force Dictionary to be readonly.

---

Miha Markic [MVP C#, INETA Country Leader for Slovenia]
Blog:http://cs.rthand.com/blogs/blog_with_righthand
Righthand .net consulting and software development
http://www.rthand.com

I am putting this code in public domain

/// <summary>
    /// Wraps an existing ICollection as read only, following the pattern of
    /// ReadOnlyCollection to simply no-op modifying functions intead of throwing
    /// Exceptions.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    [Serializable]
    public class ReadOnlyICollection<T> : ICollection<T>
    {
        /// <summary>
        ///
        /// </summary>
        private ICollection<T> coll;

        /// <summary>
        /// Initializes a new instance of the <see cref="ReadOnlyICollection&lt;T&gt;"/> class.
        /// </summary>
        /// <param name="collectionToWrap">The collection to wrap.</param>
        public ReadOnlyICollection(ICollection<T> collectionToWrap)
        {
            coll = collectionToWrap;
        }

        /// <summary>
        /// Returned a read only wrapper around the collectionToWrap.
        /// </summary>
        /// <param name="collectionToWrap">The collection to wrap.</param>
        /// <returns></returns>
        public static ReadOnlyICollection<T> AsReadOnly(ICollection<T> collectionToWrap)
        {
            return new ReadOnlyICollection<T>(collectionToWrap);
        }

        /// <summary>
        /// Add does not change a ReadOnlyICollection
        /// </summary>
        /// <param name="item">The object to add to the <see cref="T:System.Collections.Generic.ICollection`1"></see>.</param>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only.</exception>
        public void Add(T item)
        {
        }

        /// <summary>
        /// Clear does not change a ReadOnlyICollection
        /// </summary>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only. </exception>
        public void Clear()
        {
        }

        /// <summary>
        /// Determines whether the <see cref="T:System.Collections.Generic.ICollection`1"></see> contains a specific value.
        /// </summary>
        /// <param name="item">The object to locate in the <see cref="T:System.Collections.Generic.ICollection`1"></see>.</param>
        /// <returns>
        /// true if item is found in the <see cref="T:System.Collections.Generic.ICollection`1"></see>; otherwise, false.
        /// </returns>
        public bool Contains(T item)
        {
            return coll.Contains(item);
        }

        /// <summary>
        /// Copies the elements of the <see cref="T:System.Collections.Generic.ICollection`1"></see> to an <see cref="T:System.Array"></see>, starting at a particular <see cref="T:System.Array"></see> index.
        /// </summary>
        /// <param name="array">The one-dimensional <see cref="T:System.Array"></see> that is the destination of the elements copied from <see cref="T:System.Collections.Generic.ICollection`1"></see>. The <see cref="T:System.Array"></see> must have zero-based indexing.</param>
        /// <param name="arrayIndex">The zero-based index in array at which copying begins.</param>
        /// <exception cref="T:System.ArgumentOutOfRangeException">arrayIndex is less than 0.</exception>
        /// <exception cref="T:System.ArgumentNullException">array is null.</exception>
        /// <exception cref="T:System.ArgumentException">array is multidimensional.-or-arrayIndex is equal to or greater than the length of array.-or-The number of elements in the source <see cref="T:System.Collections.Generic.ICollection`1"></see> is greater than the available space from arrayIndex to the end of the destination array.-or-Type T cannot be cast automatically to the type of the destination array.</exception>
        public void CopyTo(T[] array, int arrayIndex)
        {
            coll.CopyTo(array, arrayIndex);
        }

        /// <summary>
        /// Gets the number of elements contained in the <see cref="T:System.Collections.Generic.ICollection`1"></see>.
        /// </summary>
        /// <value></value>
        /// <returns>The number of elements contained in the <see cref="T:System.Collections.Generic.ICollection`1"></see>.</returns>
        public int Count
        {
            get
            {
                return coll.Count;
            }
        }

        /// <summary>
        /// Gets a value indicating whether the <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only.
        /// </summary>
        /// <value></value>
        /// <returns>true if the <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only; otherwise, false.</returns>
        public bool IsReadOnly
        {
            get
            {
                return true;
            }
        }

        /// <summary>
        /// Remove does not change a ReadOnlyICollection
        /// </summary>
        /// <param name="item">The object to remove from the <see cref="T:System.Collections.Generic.ICollection`1"></see>.</param>
        /// <returns>
        /// true if item was successfully removed from the <see cref="T:System.Collections.Generic.ICollection`1"></see>; otherwise, false. This method also returns false if item is not found in the original <see cref="T:System.Collections.Generic.ICollection`1"></see>.
        /// </returns>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only.</exception>
        public bool Remove(T item)
        {
            return false;
        }

        /// <summary>
        /// Returns an enumerator that iterates through the collection.
        /// </summary>
        /// <returns>
        /// A <see cref="T:System.Collections.Generic.IEnumerator`1"></see> that can be used to iterate through the collection.
        /// </returns>
        public IEnumerator<T> GetEnumerator()
        {
            return coll.GetEnumerator();
        }

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>
        /// An <see cref="T:System.Collections.IEnumerator"></see> object that can be used to iterate through the collection.
        /// </returns>
        IEnumerator IEnumerable.GetEnumerator()
        {
            return coll.GetEnumerator();
        }
    }

    /// <summary>
    /// Represents a read only wrapper around a generic IDictionary. The design pattern
    /// mirrors ReadOnlyCollection, and follows the apparent pattern that write operations
    /// do not throw an exception, but simply make no change to the underlying collection.
    /// </summary>
    /// <typeparam name="TKey"></typeparam>
    /// <typeparam name="TValue"></typeparam>
    [Serializable]
    public class ReadOnlyDictionary<TKey, TValue> : IDictionary<TKey, TValue>, ICollection<KeyValuePair<TKey, TValue>>, IEnumerable<KeyValuePair<TKey, TValue>>, IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback
    {
        /// <summary>
        ///
        /// </summary>
        private IDictionary<TKey, TValue> dict;

        /// <summary>
        ///
        /// </summary>
        private IDictionary idict;

        /// <summary>
        /// Initializes a new instance of the <see cref="ReadOnlyDictionary&lt;TKey, TValue&gt;"/> class.
        /// </summary>
        /// <param name="dictionaryToWrap">The dictionary to wrap.</param>
        public ReadOnlyDictionary(IDictionary<TKey, TValue> dictionaryToWrap)
        {
            dict = dictionaryToWrap;
            idict = (IDictionary)dict;
        }

        /// <summary>
        /// Returns a read only dictionary.
        /// </summary>
        /// <param name="dictionaryToWrap">The dictionary to wrap.</param>
        /// <returns></returns>
        public static ReadOnlyDictionary<TKey, TValue> AsReadOnly(IDictionary<TKey, TValue> dictionaryToWrap)
        {
            return new ReadOnlyDictionary<TKey, TValue>(dictionaryToWrap);
        }

        /// <summary>
        /// Add does not change a read only Dictionary
        /// </summary>
        /// <param name="key">The object to use as the key of the element to add.</param>
        /// <param name="value">The object to use as the value of the element to add.</param>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.IDictionary`2"></see> is read-only.</exception>
        /// <exception cref="T:System.ArgumentException">An element with the same key already exists in the <see cref="T:System.Collections.Generic.IDictionary`2"></see>.</exception>
        /// <exception cref="T:System.ArgumentNullException">key is null.</exception>
        public void Add(TKey key, TValue value)
        {
        }

        /// <summary>
        /// Determines whether the <see cref="T:System.Collections.Generic.IDictionary`2"></see> contains an element with the specified key.
        /// </summary>
        /// <param name="key">The key to locate in the <see cref="T:System.Collections.Generic.IDictionary`2"></see>.</param>
        /// <returns>
        /// true if the <see cref="T:System.Collections.Generic.IDictionary`2"></see> contains an element with the key; otherwise, false.
        /// </returns>
        /// <exception cref="T:System.ArgumentNullException">key is null.</exception>
        public bool ContainsKey(TKey key)
        {
            return dict.ContainsKey(key);
        }

        /// <summary>
        /// Gets a read only <see cref="T:System.Collections.Generic.ICollection`1"></see> containing the keys of the <see cref="T:System.Collections.Generic.IDictionary`2"></see>.
        /// </summary>
        /// <value></value>
        /// <returns>An <see cref="T:System.Collections.Generic.ICollection`1"></see> containing the keys of the object that implements <see cref="T:System.Collections.Generic.IDictionary`2"></see>.</returns>
        public ICollection<TKey> Keys
        {
            get
            {
                return ReadOnlyICollection<TKey>.AsReadOnly(dict.Keys);
            }
        }

        /// <summary>
        /// Remove does not change a read only Dictionary
        /// </summary>
        /// <param name="key">The key of the element to remove.</param>
        /// <returns>
        /// true if the element is successfully removed; otherwise, false.  This method also returns false if key was not found in the original <see cref="T:System.Collections.Generic.IDictionary`2"></see>.
        /// </returns>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.IDictionary`2"></see> is read-only.</exception>
        /// <exception cref="T:System.ArgumentNullException">key is null.</exception>
        public bool Remove(TKey key)
        {
            return false;
        }

        /// <summary>
        /// Tries the get value.
        /// </summary>
        /// <param name="key">The key.</param>
        /// <param name="value">The value.</param>
        /// <returns></returns>
        public bool TryGetValue(TKey key, out TValue value)
        {
            return dict.TryGetValue(key, out value);
        }

        /// <summary>
        /// Gets an <see cref="T:System.Collections.Generic.ICollection`1"></see> containing the values in the <see cref="T:System.Collections.Generic.IDictionary`2"></see>.
        /// </summary>
        /// <value></value>
        /// <returns>An <see cref="T:System.Collections.Generic.ICollection`1"></see> containing the values in the object that implements <see cref="T:System.Collections.Generic.IDictionary`2"></see>.</returns>
        public ICollection<TValue> Values
        {
            get
            {
                return ReadOnlyICollection<TValue>.AsReadOnly(dict.Values);
            }
        }

        /// <summary>
        /// Gets the <see cref="TValue"/> with the specified key. Set
        /// does not change a read only Dictionary
        /// </summary>
        /// <value></value>
        public TValue this[TKey key]
        {
            get
            {
                return dict[key];
            }
            set
            {
            }
        }

        /// <summary>
        /// Add does not change a read only Dictionary
        /// </summary>
        /// <param name="item">The object to add to the <see cref="T:System.Collections.Generic.ICollection`1"></see>.</param>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only.</exception>
        public void Add(KeyValuePair<TKey, TValue> item)
        {
        }

        /// <summary>
        /// Clear does not change a read only Dictionary.
        /// </summary>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only. </exception>
        public void Clear()
        {
        }

        /// <summary>
        /// Determines whether the <see cref="T:System.Collections.Generic.ICollection`1"></see> contains a specific value.
        /// </summary>
        /// <param name="item">The object to locate in the <see cref="T:System.Collections.Generic.ICollection`1"></see>.</param>
        /// <returns>
        /// true if item is found in the <see cref="T:System.Collections.Generic.ICollection`1"></see>; otherwise, false.
        /// </returns>
        public bool Contains(KeyValuePair<TKey, TValue> item)
        {
            return dict.Contains(item);
        }

        /// <summary>
        /// Copies the elements of the <see cref="T:System.Collections.Generic.ICollection`1"></see> to an <see cref="T:System.Array"></see>, starting at a particular <see cref="T:System.Array"></see> index.
        /// </summary>
        /// <param name="array">The one-dimensional <see cref="T:System.Array"></see> that is the destination of the elements copied from <see cref="T:System.Collections.Generic.ICollection`1"></see>. The <see cref="T:System.Array"></see> must have zero-based indexing.</param>
        /// <param name="arrayIndex">The zero-based index in array at which copying begins.</param>
        /// <exception cref="T:System.ArgumentOutOfRangeException">arrayIndex is less than 0.</exception>
        /// <exception cref="T:System.ArgumentNullException">array is null.</exception>
        /// <exception cref="T:System.ArgumentException">array is multidimensional.-or-arrayIndex is equal to or greater than the length of array.-or-The number of elements in the source <see cref="T:System.Collections.Generic.ICollection`1"></see> is greater than the available space from arrayIndex to the end of the destination array.-or-Type T cannot be cast automatically to the type of the destination array.</exception>
        public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
        {
            dict.CopyTo(array, arrayIndex);
        }

        /// <summary>
        /// Gets the number of elements contained in the <see cref="T:System.Collections.Generic.ICollection`1"></see>.
        /// </summary>
        /// <value></value>
        /// <returns>The number of elements contained in the <see cref="T:System.Collections.Generic.ICollection`1"></see>.</returns>
        public int Count
        {
            get
            {
                return dict.Count;
            }
        }

        /// <summary>
        /// Gets a value indicating whether the <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only.
        /// </summary>
        /// <value></value>
        /// <returns>true if the <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only; otherwise, false.</returns>
        public bool IsReadOnly
        {
            get
            {
                return true;
            }
        }

        /// <summary>
        /// Remove does not change a read only Dictionary
        /// </summary>
        /// <param name="item">The object to remove from the <see cref="T:System.Collections.Generic.ICollection`1"></see>.</param>
        /// <returns>
        /// true if item was successfully removed from the <see cref="T:System.Collections.Generic.ICollection`1"></see>; otherwise, false. This method also returns false if item is not found in the original <see cref="T:System.Collections.Generic.ICollection`1"></see>.
        /// </returns>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"></see> is read-only.</exception>
        public bool Remove(KeyValuePair<TKey, TValue> item)
        {
            return false;
        }

        /// <summary>
        /// Returns an enumerator that iterates through the collection.
        /// </summary>
        /// <returns>
        /// A <see cref="T:System.Collections.Generic.IEnumerator`1"></see> that can be used to iterate through the collection.
        /// </returns>
        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
        {
            return dict.GetEnumerator();
        }

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>
        /// An <see cref="T:System.Collections.IEnumerator"></see> object that can be used to iterate through the collection.
        /// </returns>
        IEnumerator IEnumerable.GetEnumerator()
        {
            return idict.GetEnumerator();
        }

        /// <summary>
        /// Add does not change a read only Dictionary
        /// </summary>
        /// <param name="key">The <see cref="T:System.Object"></see> to use as the key of the element to add.</param>
        /// <param name="value">The <see cref="T:System.Object"></see> to use as the value of the element to add.</param>
        /// <exception cref="T:System.ArgumentException">An element with the same key already exists in the <see cref="T:System.Collections.IDictionary"></see> object. </exception>
        /// <exception cref="T:System.ArgumentNullException">key is null. </exception>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.IDictionary"></see> is read-only.-or- The <see cref="T:System.Collections.IDictionary"></see> has a fixed size. </exception>
        public void Add(object key, object value)
        {
        }

        /// <summary>
        /// Determines whether the <see cref="T:System.Collections.IDictionary"></see> object contains an element with the specified key.
        /// </summary>
        /// <param name="key">The key to locate in the <see cref="T:System.Collections.IDictionary"></see> object.</param>
        /// <returns>
        /// true if the <see cref="T:System.Collections.IDictionary"></see> contains an element with the key; otherwise, false.
        /// </returns>
        /// <exception cref="T:System.ArgumentNullException">key is null. </exception>
        public bool Contains(object key)
        {
            return idict.Contains(key);
        }

        /// <summary>
        /// Returns an <see cref="T:System.Collections.IDictionaryEnumerator"></see> object for the <see cref="T:System.Collections.IDictionary"></see> object.
        /// </summary>
        /// <returns>
        /// An <see cref="T:System.Collections.IDictionaryEnumerator"></see> object for the <see cref="T:System.Collections.IDictionary"></see> object.
        /// </returns>
        IDictionaryEnumerator IDictionary.GetEnumerator()
        {
            return idict.GetEnumerator();
        }

        /// <summary>
        /// Gets a value indicating whether the <see cref="T:System.Collections.IDictionary"></see> object has a fixed size.
        /// </summary>
        /// <value></value>
        /// <returns>true if the <see cref="T:System.Collections.IDictionary"></see> object has a fixed size; otherwise, false.</returns>
        public bool IsFixedSize
        {
            get
            {
                return idict.IsFixedSize;
            }
        }

        /// <summary>
        /// Gets an <see cref="T:System.Collections.Generic.ICollection`1"></see> containing the keys of the <see cref="T:System.Collections.Generic.IDictionary`2"></see>.
        /// </summary>
        /// <value></value>
        /// <returns>An <see cref="T:System.Collections.Generic.ICollection`1"></see> containing the keys of the object that implements <see cref="T:System.Collections.Generic.IDictionary`2"></see>.</returns>
        ICollection IDictionary.Keys
        {
            get
            {
                return idict.Keys;
            }
        }

        /// <summary>
        /// Remove does not affect a read only Dictionary
        /// </summary>
        /// <param name="key">The key of the element to remove.</param>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.IDictionary"></see> object is read-only.-or- The <see cref="T:System.Collections.IDictionary"></see> has a fixed size. </exception>
        /// <exception cref="T:System.ArgumentNullException">key is null. </exception>
        public void Remove(object key)
        {
        }

        /// <summary>
        /// Gets an <see cref="T:System.Collections.Generic.ICollection`1"></see> containing the values in the <see cref="T:System.Collections.Generic.IDictionary`2"></see>.
        /// </summary>
        /// <value></value>
        /// <returns>An <see cref="T:System.Collections.Generic.ICollection`1"></see> containing the values in the object that implements <see cref="T:System.Collections.Generic.IDictionary`2"></see>.</returns>
        ICollection IDictionary.Values
        {
            get
            {
                return idict.Values;
            }
        }

        /// <summary>
        /// Gets the <see cref="System.Object"/> with the specified key. Set
        /// does not affect a ReadOnlyDictionary
        /// </summary>
        /// <value></value>
        public object this[object key]
        {
            get
            {
                return idict[key];
            }
            set
            {
            }
        }

        /// <summary>
        /// Copies the elements of the <see cref="T:System.Collections.ICollection"></see> to an <see cref="T:System.Array"></see>, starting at a particular <see cref="T:System.Array"></see> index.
        /// </summary>
        /// <param name="array">The one-dimensional <see cref="T:System.Array"></see> that is the destination of the elements copied from <see cref="T:System.Collections.ICollection"></see>. The <see cref="T:System.Array"></see> must have zero-based indexing.</param>
        /// <param name="index">The zero-based index in array at which copying begins.</param>
        /// <exception cref="T:System.ArgumentNullException">array is null. </exception>
        /// <exception cref="T:System.ArgumentOutOfRangeException">index is less than zero. </exception>
        /// <exception cref="T:System.ArgumentException">array is multidimensional.-or- index is equal to or greater than the length of array.-or- The number of elements in the source <see cref="T:System.Collections.ICollection"></see> is greater than the available space from index to the end of the destination array. </exception>
        /// <exception cref="T:System.InvalidCastException">The type of the source <see cref="T:System.Collections.ICollection"></see> cannot be cast automatically to the type of the destination array. </exception>
        public void CopyTo(Array array, int index)
        {
        }

        /// <summary>
        /// Gets a value indicating whether access to the <see cref="T:System.Collections.ICollection"></see> is synchronized (thread safe).
        /// </summary>
        /// <value></value>
        /// <returns>true if access to the <see cref="T:System.Collections.ICollection"></see> is synchronized (thread safe); otherwise, false.</returns>
        public bool IsSynchronized
        {
            get
            {
                return idict.IsSynchronized;
            }
        }

        /// <summary>
        /// Gets an object that can be used to synchronize access to the <see cref="T:System.Collections.ICollection"></see>.
        /// </summary>
        /// <value></value>
        /// <returns>An object that can be used to synchronize access to the <see cref="T:System.Collections.ICollection"></see>.</returns>
        public object SyncRoot
        {
            get
            {
                return idict.SyncRoot;
            }
        }

        /// <summary>
        /// Runs when the entire object graph has been deserialized.
        /// </summary>
        /// <param name="sender">The object that initiated the callback. The functionality for this parameter is not currently implemented.</param>
        public void OnDeserialization(object sender)
        {
            IDeserializationCallback callback = dict as IDeserializationCallback;
            callback.OnDeserialization(sender);
        }

        /// <summary>
        /// Populates a <see cref="T:System.Runtime.Serialization.SerializationInfo"></see> with the data needed to serialize the target object.
        /// </summary>
        /// <param name="info">The <see cref="T:System.Runtime.Serialization.SerializationInfo"></see> to populate with data.</param>
        /// <param name="context">The destination (see <see cref="T:System.Runtime.Serialization.StreamingContext"></see>) for this serialization.</param>
        /// <exception cref="T:System.Security.SecurityException">The caller does not have the required permission. </exception>
        public void GetObjectData(SerializationInfo info, StreamingContext context)
        {
            ISerializable serializable = dict as ISerializable;
            serializable.GetObjectData(info, context);
        }
    }

I would just add that you need to include the following using statements:

using System.Collections;
using System.Collections.ObjectModel;
using System.Runtime.Serialization;