C# Where

The where clause is used to specify constraints on the types that can be acclimated as arguments for a blazon parameter defined in a generic declaration. For example, you can declare a generic class such that the type parameter T implements the IComparable<T> interface:

// cs_where.cs
// compile with: /target:library
using System;

class MyClassy<T, U>
    where T : class
    where U : struct
{
}

 

The new() Constraint lets the compiler know that any type argument supplied must have an accessible parameterless--or default-- constructor.

For example:

// cs_where_2.cs
// compile with: /target:library
using System;
public class MyGenericClass <T> where T: IComparable, new()
{
    // The following line is not possible without new() constraint:
    T item = new T();
}

With multiple type parameters, use one where clause for each type parameter

For example:

// cs_where_3.cs
// compile with: /target:library
using System;
using System.Collections;

interface MyI
{
}

class Dictionary<TKey,TVal>
    where TKey: IComparable, IEnumerable
    where TVal: MyI
{
    public void Add(TKey key, TVal val)
    {
    }
}