Casting a C# Object From Its Parent

Have you ever tried to do something akin to the following?

[Fact]
public void ConvertClassToSubClass_Converts()
{
    // Arrange
    var parentClass = new SimpleTestClass();
    parentClass.Property1 = "test";

    // Act
    var childClass = parentClass as SimpleTestSubClass;

    // Assert
    Assert.Equal("test", childClass.Property1);
}

This is a simple Xunit (failing) test. The reason it fails is because you (or I) am trying to cast a general type to a specific, and C# is complaining that this may not be possible; consequently, you will get null (or for a hard cast, you'll get an InvalidCastException).

Okay, that makes sense. After all, parentClass   could actually be a  SimpleTestSubClass2   and, as a result, C# is being safe because there's (presumably, I don't work for MS) too many possibilities for edge cases.

This is, however, a solvable problem; there are a few ways to do it, but you can simply use reflection:

public TNewClass CastAsClass<TNewClass>() where TNewClass : class
{

    var newObject = Activator.CreateInstance<TNewClass>();
    var newProps = typeof(TNewClass).GetProperties();

    foreach (var prop in newProps)
    {
        if (!prop.CanWrite) continue;

        var existingPropertyInfo = typeof(TExistingClass).GetProperty(prop.Name);
        if (existingPropertyInfo == null || !existingPropertyInfo.CanRead) continue;
        var value = existingPropertyInfo.GetValue(_existingClass);

        prop.SetValue(newObject, value, null);
    }

    return newObject;
}

This code will effectively transfer any class over to any other class.

If you'd rather use an existing library, you can always use this one . It's also o pen sourced on GitHib .