Export (0) Print
Expand All
Expand Minimize

Compiler Error CS0304

Cannot create an instance of the variable type 'type' because it does not have the new() constraint

When you implement a generic class, and you want to use the new keyword to create a new instance of any type that is supplied for a type parameter T, you must apply the new() constraint to T in the class declaration, as shown in the following example.

class C<T> where T : new()

The new() constraint enforces type safety by guaranteeing that any concrete type that is supplied for T has a default, parameterless constructor. CS0304 occurs if you attempt to use the new operator in the body of the class to create an instance of type parameter T when T does not specify the new() constraint. On the client side, if code attempts to instantiate the generic class with a type that has no default constructor, that code will generate Compiler Error CS0310.

The following example generates CS0304.

// CS0304.cs
// Compile with: /target:library.
class C<T>
{
    // The following line generates CS0304.
    T t = new T();
}

The new operator also is not allowed in methods of the class.

// Compile with: /target:library.
class C<T>
{
    public void ExampleMethod()
    {
        // The following line generates CS0304.
        T t = new T();
    }
}

To avoid the error, declare the class by using the new() constraint, as shown in the following example.

// Compile with: /target:library.
class C<T> where T : new()
{
    T t = new T();

    public void ExampleMethod()
    {
        T t = new T();
    }
}
Show:
© 2014 Microsoft