How to call protected constructor in c#?

Question

How to call protected constructor?

public class Foo{
  public Foo(a lot of arguments){}
  protected Foo(){}
}
var foo=???

This obviously fails

Answer 1

If you want to avoid repeated reflection cost, you can use expressions. Here is an example of calling a private constructor with a string value.

    private static Func CreateInstanceFunc()
    {
        var flags = BindingFlags.NonPublic | BindingFlags.Instance;
        var ctor = typeof(T).GetConstructors(flags).Single(
            ctors =>
            {
                var parameters = ctors.GetParameters();
                return parameters.Length == 1 && parameters[0].ParameterType == typeof(string);
            });
        var value = Expression.Parameter(typeof(string), "value");
        var body = Expression.New(ctor, value);
        var lambda = Expression.Lambda>(body, value);

        return lambda.Compile();
    }

Save the cost of compiling the function multiple times by storing it in a static field.

private static readonly Lazy> CreateInstance = new Lazy>(CreateInstanceFunc);

Now you can create the object with

CreateInstance.Value("Hello")