‘is’ versus try cast with null check

Because there’s only one cast. Compare this:

if (myObj.myProp is MyType) // cast #1
{
    var myObjRef = (MyType)myObj.myProp; // needs to be cast a second time
                                         // before using it as a MyType
    ...
}

to this:

var myObjRef = myObj.myProp as MyType; // only one cast
if (myObjRef != null)
{
    // myObjRef is already MyType and doesn't need to be cast again
    ...
}

C# 7.0 supports a more compact syntax using pattern matching:

if (myObj.myProp is MyType myObjRef)
{
    ...
}