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Compiler Error CS0304

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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 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 parameterless 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();  
    }  
}

See also