How to return a value using constructor in c#?

The constructor does return a value - the type being constructed...

A constructor is not supposed to return any other type of value.

When you validate in a constructor, you should throw exceptions if the passed in values are invalid.

public class MyType
{
    public MyType(int toValidate)
    {
      if (toValidate < 0)
      {
        throw new ArgumentException("toValidate should be positive!");
      }
    }
}

A constructor returns the type being instantiated, and, if necessary, can throw an exception. Perhaps a better solution would be to add a static method to try creating an instance of your class:

public static bool TryCreatingMyClass(out MyClass result, out string message)
{
    // Set the value of result and message, return true if success, false if failure
}