1) Why:
As described before, the problem is that Dictionary requires an implementation of the Object.GetHashCode()
method. null
does not have an implementation, therefore no hash code associated.
2) Solution: I have used a solution similar to a NullObject pattern using generics that enables you to use the dictionary seamlessly (no need for a different dictionary implementation).
You can will use it, like this:
var dict = new Dictionary<NullObject<Type>, string>();
dict[typeof(int)] = "int type";
dict[typeof(string)] = "string type";
dict[null] = "null type";
Assert.AreEqual("int type", dict[typeof(int)]);
Assert.AreEqual("string type", dict[typeof(string)]);
Assert.AreEqual("null type", dict[null]);
You just need to create this struct once in a lifetime :
public struct NullObject<T>
{
[DefaultValue(true)]
private bool isnull;// default property initializers are not supported for structs
private NullObject(T item, bool isnull) : this()
{
this.isnull = isnull;
this.Item = item;
}
public NullObject(T item) : this(item, item == null)
{
}
public static NullObject<T> Null()
{
return new NullObject<T>();
}
public T Item { get; private set; }
public bool IsNull()
{
return this.isnull;
}
public static implicit operator T(NullObject<T> nullObject)
{
return nullObject.Item;
}
public static implicit operator NullObject<T>(T item)
{
return new NullObject<T>(item);
}
public override string ToString()
{
return (Item != null) ? Item.ToString() : "NULL";
}
public override bool Equals(object obj)
{
if (obj == null)
return this.IsNull();
if (!(obj is NullObject<T>))
return false;
var no = (NullObject<T>)obj;
if (this.IsNull())
return no.IsNull();
if (no.IsNull())
return false;
return this.Item.Equals(no.Item);
}
public override int GetHashCode()
{
if (this.isnull)
return 0;
var result = Item.GetHashCode();
if (result >= 0)
result++;
return result;
}
}