Jonathan Yong

Reference type constraint allow you to specify that the type parameter must be of reference type.

class GenericReferenceType<T> where T : class
{
    public T Value;
}
 
static void InstantiateRefType()
{
    // This code will compile fine.
    GenericReferenceType<System.Text.StringBuilder> vt =
        new GenericReferenceType<StringBuilder>();
 
    // This code will give compile error 
    //because Point is not reference type.
    GenericReferenceType<System.Drawing.Point> vt2 =
        new GenericReferenceType<System.Drawing.Point>();
}    

In the above sample, GenericReferenceType can only accept a reference type as its type parameter.
where keyword is used to specify a generic constraint in C#. In the above code sample, I specify that
type parameter T has to be of reference type (indicate by where T : class).

Labels: csharp, DotNet, Generics