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Compiler Error CS0417

Visual Studio 2008

'identifier': cannot provide arguments when creating an instance of a variable type

This error occurs if a call to the new operator on a type parameter has arguments. The only constructor that can be called using the new operator on an unknown parameter type is a constructor with no arguments. If you need to call another constructor, consider using a class type constraint or interface constraint.

The following example generates CS0417:

Copy 
// CS0417
class C<T> where T : new()
{
    T type = new T(1);   // CS0417
}
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Constructor constraint only works with default constructor

You cannot call other constructors than the default constructor. Something like

class TBase
{
  private int ii;
  public TBase(int i)
  {
   ii = i;
  }
}
  

class C<T> where T : TBase
{
  T type = new T(42);
}

doesn't work!


CS Team edit: Correct. Your version will cause CS0304. Then, if you change the declaration of C to include the new constraint, you will get this error.

class C<T> where T : TBase, new()
{
    T type = new T(42);
}
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You can work around this issue using Activator.CreateInstance 
public class DataContextFactory<T> where T : DataContext
{
        public static T GetDataContext()
        {
            T retval = (T) Activator.CreateInstance(typeof(T), 
                new object[] { GetConnectionString() });
            return retval;
        }
..
..
}
History