I want to write a .NET program calling Mathematica as a computing engine. But here is a problem that argument a1 is not a constant and its value depends on a1=str1+str2. There is a mathematica function(fun1) I need to call and the argument of fun1 is a string. In Mathematica, I program like fun1["@150"]. But now, the problem is coming. Firstly, I think I can call fun1 like this:

Dim ml As IkernelLink = MathLinkFactory.CreateKernelLink()
ml.Evaluate("fun1[a1]")

But I found it is wrong, because in Mathematica the argument of fun1 is essentially a string. But here we also can not use ml.Evaluate("fun1["a1"]"). How can I solve this problem?