Class Mapping Error: 'T' must be a non-abstract type with a public parameterless constructor

Question

While mapping class i am getting error \'T\' must be a non-abstract type with a public parameterless constructor in order to use it as parameter \'T\' in the generic type or

Answer 1

I had the same issue. I should have read the message before Googling it. I needed to add a parameterless constructor ... :-)

public MyClass() {
  //stuff
}

Answer 2

The constraints must apply to every type in the chain; hence you need:

public abstract class SqlReaderBase<T> : ConnectionProvider where T : new()

Without this, you can't satisfy the constraint for T in:

protected abstract MapperBase<T> GetMapper();

or

MapperBase<T> mapper = GetMapper();

since MapperBase<> is only usable when T has : new()

Answer 3

The problem is that you're trying to use the T from SqlReaderBase as the type argument for MapperBase - but you don't have any constraints on that T.

Try changing your SqlReaderBase declaration to this:

public abstract class SqlReaderBase<T> : ConnectionProvider
    where T : new()

Here's a shorter example which demonstrates the same issue:

class Foo<T>
{
    Bar<T> bar;
}

class Bar<T> where T : new()
{
}

The fix is to change Foo<T>'s declaration to:

class Foo<T> where T : new()

Then the compiler will know that the T from Foo is a valid type argument for Bar.