Override Dictionary.Add

Question

I need to know how to override the Add-method of a certain Dictionary in a certain static class. Any suggestions?

If it matters, the dictionary looks like this:

Answer 1

You can't override the Add method of Dictionary<,> since it's non virtual. You can hide it by adding a method with the same name/signature in the derived class, but hiding isn't the same as overriding. If somebody casts to the base class he will still call the wrong Add.

The correct way to do this is to create your own class that implements IDictionary<,> (the interface) but has a Dictionary<,> (the class) instead of being a Dictionary<,>.

class MyDictionary<TKey,TValue>:IDictionary<TKey,TValue>
{
  private Dictionary<TKey,TValue> backingDictionary;

  //Implement the interface here
  //Delegating most of the logic to your backingDictionary
  ...
}

Answer 2

I went with CodesInChaos's solution. Since there's a bit of overhead involved in implementing IDictionary<TKey, TValue>, here is a virtual implementation for general consumption. I'm including some comments and an example below for optional reading. Let me know if I should fix or improve something. I haven't done much testing, but it seems to work for overriding the add method.

using System.Collections;
using System.Collections.Generic;

/// <summary>
/// Represents a collection of keys and values. This is an abstract base-class wrapping a <see cref="T:System.Collections.Generic.Dictionary`2"/> with 
/// virtual method that can be overridden. This class can be used to override the default functionality of <see cref="T:System.Collections.Generic.Dictionary`2"/>.
/// </summary>
/// <typeparam name="TKey">The type of the keys in the dictionary.</typeparam>
/// <typeparam name="TValue">The type of the values in the dictionary.</typeparam>
public class VirtualDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
    protected IDictionary<TKey, TValue> wrappedDictionary;

    /// <summary>
    /// Initializes a new instance of the <see cref="VirtualDictionary{TKey,TValue}"/> class that is empty, has the default initial capacity, and uses the default equality comparer for the key type.
    /// </summary>
    public VirtualDictionary()
    {
        wrappedDictionary = new Dictionary<TKey, TValue>();
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="VirtualDictionary{TKey,TValue}"/> class that is empty, has the specified initial capacity, and uses the default equality comparer for the key type.
    /// </summary>
    /// <param name="capacity">The initial number of elements that the <see cref="T:System.Collections.Generic.VirtualDictionary`2"/> can contain.</param><exception cref="T:System.ArgumentOutOfRangeException"><paramref name="capacity"/> is less than 0.</exception>
    public VirtualDictionary(int capacity)
    {
        wrappedDictionary = new Dictionary<TKey, TValue>(capacity);
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="VirtualDictionary{TKey,TValue}"/> class that is empty, has the default initial capacity, and uses the specified <see cref="T:System.Collections.Generic.IEqualityComparer`1"/>.
    /// </summary>
    /// <param name="comparer">The <see cref="T:System.Collections.Generic.IEqualityComparer`1"/> implementation to use when comparing keys, or null to use the default <see cref="T:System.Collections.Generic.EqualityComparer`1"/> for the type of the key.</param>
    public VirtualDictionary(IEqualityComparer<TKey> comparer)
    {
        wrappedDictionary = new Dictionary<TKey, TValue>(comparer);
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="VirtualDictionary{TKey,TValue}"/> class that is empty, has the specified initial capacity, and uses the specified <see cref="T:System.Collections.Generic.IEqualityComparer`1"/>.
    /// </summary>
    /// <param name="capacity">The initial number of elements that the <see cref="VirtualDictionary{TKey,TValue}"/> can contain.</param><param name="comparer">The <see cref="T:System.Collections.Generic.IEqualityComparer`1"/> implementation to use when comparing keys, or null to use the default <see cref="T:System.Collections.Generic.EqualityComparer`1"/> for the type of the key.</param><exception cref="T:System.ArgumentOutOfRangeException"><paramref name="capacity"/> is less than 0.</exception>
    public VirtualDictionary(int capacity, IEqualityComparer<TKey> comparer)
    {
        wrappedDictionary = new Dictionary<TKey, TValue>(capacity, comparer);
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="VirtualDictionary{TKey,TValue}"/> class that contains elements copied from the specified <see cref="T:System.Collections.Generic.IDictionary`2"/> and uses the default equality comparer for the key type.
    /// </summary>
    /// <param name="dictionary">The <see cref="T:System.Collections.Generic.IDictionary`2"/> whose elements are copied to the new <see cref="VirtualDictionary{TKey,TValue}"/>.</param><exception cref="T:System.ArgumentNullException"><paramref name="dictionary"/> is null.</exception><exception cref="T:System.ArgumentException"><paramref name="dictionary"/> contains one or more duplicate keys.</exception>
    public VirtualDictionary(IDictionary<TKey, TValue> dictionary)
    {
        wrappedDictionary = new Dictionary<TKey, TValue>(dictionary);
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="VirtualDictionary{TKey,TValue}"/> class that contains elements copied from the specified <see cref="T:System.Collections.Generic.IDictionary`2"/> and uses the specified <see cref="T:System.Collections.Generic.IEqualityComparer`1"/>.
    /// </summary>
    /// <param name="dictionary">The <see cref="T:System.Collections.Generic.IDictionary`2"/> whose elements are copied to the new <see cref="VirtualDictionary{TKey,TValue}"/>.</param><param name="comparer">The <see cref="T:System.Collections.Generic.IEqualityComparer`1"/> implementation to use when comparing keys, or null to use the default <see cref="T:System.Collections.Generic.EqualityComparer`1"/> for the type of the key.</param><exception cref="T:System.ArgumentNullException"><paramref name="dictionary"/> is null.</exception><exception cref="T:System.ArgumentException"><paramref name="dictionary"/> contains one or more duplicate keys.</exception>
    public VirtualDictionary(IDictionary<TKey, TValue> dictionary, IEqualityComparer<TKey> comparer)
    {
        wrappedDictionary = new Dictionary<TKey, TValue>(dictionary, comparer);
    }

    /// <summary>
    /// Adds an element with the provided key and value to the <see cref="T:System.Collections.Generic.IDictionary`2"/>.
    /// </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.ArgumentNullException"><paramref name="key"/> is null.</exception><exception cref="T:System.ArgumentException">An element with the same key already exists in the <see cref="T:System.Collections.Generic.IDictionary`2"/>.</exception><exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.IDictionary`2"/> is read-only.</exception>
    public virtual void Add(TKey key, TValue value)
    {
        wrappedDictionary.Add(key, value);
    }

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

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

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

    /// <summary>
    /// Gets the value associated with the specified key.
    /// </summary>
    /// <returns>
    /// true if the object that implements <see cref="T:System.Collections.Generic.IDictionary`2"/> contains an element with the specified key; otherwise, false.
    /// </returns>
    /// <param name="key">The key whose value to get.</param><param name="value">When this method returns, the value associated with the specified key, if the key is found; otherwise, the default value for the type of the <paramref name="value"/> parameter. This parameter is passed uninitialized.</param><exception cref="T:System.ArgumentNullException"><paramref name="key"/> is null.</exception>
    public virtual bool TryGetValue(TKey key, out TValue value)
    {
        return wrappedDictionary.TryGetValue(key, out value);
    }

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

    /// <summary>
    /// Gets or sets the element with the specified key.
    /// </summary>
    /// <returns>
    /// The element with the specified key.
    /// </returns>
    /// <param name="key">The key of the element to get or set.</param><exception cref="T:System.ArgumentNullException"><paramref name="key"/> is null.</exception><exception cref="T:System.Collections.Generic.KeyNotFoundException">The property is retrieved and <paramref name="key"/> is not found.</exception><exception cref="T:System.NotSupportedException">The property is set and the <see cref="T:System.Collections.Generic.IDictionary`2"/> is read-only.</exception>
    public virtual TValue this[TKey key]
    {
        get
        {
            return wrappedDictionary[key];
        }
        set
        {
            wrappedDictionary[key] = value;
        }
    }

    /// <summary>
    /// Adds an item to the <see cref="T:System.Collections.Generic.ICollection`1"/>.
    /// </summary>
    /// <param name="item">The object to add to the <see cref="T:System.Collections.Generic.ICollection`1"/>.</param><exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only.</exception>
    public virtual void Add(KeyValuePair<TKey, TValue> item)
    {
        wrappedDictionary.Add(item);
    }

    /// <summary>
    /// Removes all items from the <see cref="T:System.Collections.Generic.ICollection`1"/>.
    /// </summary>
    /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only. </exception>
    public virtual void Clear()
    {
        wrappedDictionary.Clear();
    }

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

    /// <summary>
    /// Copies the elements of the <see cref="T:System.Collections.Generic.ICollection`1"/> to an <see cref="T:System.Array"/>, starting at a particular <see cref="T:System.Array"/> index.
    /// </summary>
    /// <param name="array">The one-dimensional <see cref="T:System.Array"/> that is the destination of the elements copied from <see cref="T:System.Collections.Generic.ICollection`1"/>. The <see cref="T:System.Array"/> must have zero-based indexing.</param><param name="arrayIndex">The zero-based index in <paramref name="array"/> at which copying begins.</param><exception cref="T:System.ArgumentNullException"><paramref name="array"/> is null.</exception><exception cref="T:System.ArgumentOutOfRangeException"><paramref name="arrayIndex"/> is less than 0.</exception><exception cref="T:System.ArgumentException">The number of elements in the source <see cref="T:System.Collections.Generic.ICollection`1"/> is greater than the available space from <paramref name="arrayIndex"/> to the end of the destination <paramref name="array"/>.</exception>
    public virtual void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
    {
        wrappedDictionary.CopyTo(array, arrayIndex);
    }

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

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

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

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

    /// <summary>
    /// Returns an enumerator that iterates through a collection.
    /// </summary>
    /// <returns>
    /// An <see cref="T:System.Collections.IEnumerator"/> object that can be used to iterate through the collection.
    /// </returns>
    /// <filterpriority>2</filterpriority>
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

My use case was to be able to run the same code to manipulate a DataTable and a kendo datasource result which is IDictionary. So, I wrapped the DataTable with my custom IDictionary such that updates to the IDictionary were made on the DataRow. Maybe someone will find that useful and should serve as an example implementation.

public class DataRowDictionaryWrapper : VirtualDictionary<string, object>
{
    private DataRow row;

    public DataRowDictionaryWrapper(DataRow row)
    {
        this.row = row;
        this.wrappedDictionary = row.Table.Columns.Cast<DataColumn>().ToDictionary(key => key.ColumnName, c => row[c]);
    }

    public override void Add(string key, object value)
    {
        DataColumn col = row.Table.Columns[key];
        if (col == null)
        {
            col = new DataColumn(key);
            row.Table.Columns.Add(col);
        }
        row[col] = value;

        base[key] = value;
    }
}

public class DataTableDictionaryWrapper : IEnumerable<IDictionary<string, object>>
{
    private DataTable dt;

    public DataTableDictionaryWrapper(DataTable dt)
    {
        this.dt = dt;
    }

    public IEnumerator<IDictionary<string, object>> GetEnumerator()
    {
        foreach (DataRow row in dt.Rows)
        {
            yield return new DataRowDictionaryWrapper(row);
        }
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return this.GetEnumerator();
    }
}

Answer 3

It's actually simpler to use the "new" keyword. I do this often when my objects contain thier own unique ID value.

// Warning Air Code - Subject to bonehead mistakes

internal sealed class SomeDict : Dictionary<myKeyType, myObjectType>
{
    public new void Add(myKeyType key, myObjectType value)
    {
        this.Add(value);
    }

    internal void Add(myObjectType value)
    {
        base.Add(value.Id, value);
    }

}

Answer 4

In some scenarios, where TKey is a property of TValue, you may want to consider using KeyedCollection<TKey, TValue>, which allows you to override InsertItem, SetItem, RemoveItem, and ClearItems.
It uses a dictionary internally (unless specified otherwise via the threshold constructor parameter), but also manages a collection.

I Haven't benchmarked or compared performance, there might be a performance difference so use carefully.

Consider voting on this feature suggestion on the .NET Core repo.

Answer 5

Another "quick and dirty" fix

If you need to add some extra logic to the Add method, you can create your own method, with a slightly different signature, and use THAT in your code.

public class MyOwnDictionary<TKey, TValue>: Dictionary<TKey, TValue>
{
    public void AddPlus(TKey key, TValue value)
    {
        //some custom logic here

        base.Add(key,value);
    }
}

(but this can lead to maintenance difficulties)