c# shorthand for if not null then assign value

There are a couple!

The ternary operator:

testvar2 = testVar1 != null ? testvar1 : testvar2;

Would be exactly the same logic.

Or, as commented you can use the null coalescing operator:

testVar2 = testVar1 ?? testVar2

(although now that’s been commented as well)

Or a third option: Write a method once and use it how you like:

public static class CheckIt
{
    public static void SetWhenNotNull(string mightBeNull,ref string notNullable)
    {
        if (mightBeNull != null)
        {
            notNullable = mightBeNull;
        }
    }
}  

And call it:

CheckIt.SetWhenNotNull(test1, ref test2);