When initializing in C# constructors what's better: initializer lists or assignment?

The first one is not legal in C#. The only two items that can appear after the colon in a constructor are base and this.

So I'd go with the second one.

Did you mean C++ instead of C#?

For C++, initializer lists are better than assignment for a couple of reasons:

• For POD types (int, float, etc..), the optimizer can often do a more efficient memcpy behind the scenes when you provide initializers for the data.
• For non-POD types (objects), you gain efficiency by only doing one construction. With assignment in the constructor, the compiler must first construct your object, then assign it in a separate step (this applies to POD types as well).

As of C# 7.0, there is a way to streamline this with expression bodies:

A(String filename) => _filename = filename;

(Looks better with two fields though):

A(String filename, String extension) => (_filename, _extension) = (filename, extension);