How to fake a datacontext method with TypeMock Isolator

Question

I want to test a class that uses Linq to SQL. I have faked the datacontext with TypeMock Isolator, but the datacontext also has a function that I don't know how to fake. This function is used in Linq to Sql-queries.

The function passes two arguments (int? a, string b), and returns an integer; DC.MyMethod(int? a, string b)

How to I fake this?

//Fake datacontext
var fakeDC = Isolate.Fake.Instance<MyDataContext>();

//Fake --> this doesn't work
Isolate.WhenCalled((int? a, string b) => fakeDC.MyFunction(a,b).... ?

Hope anyone can help

Answer 1

Isolator by default ignores arguments passed to functions. To fake yours, you could simply use:

Isolate.WhenCalled(() => fakeDC.MyFunction(null, null)).WillReturn(...)

If you need to make sure it was called with specific arguments, add WithExactArguments(), like this:

int? id = 10;
string name = "David";

Isolate.WhenCalled(() => fakeDC.MyFunction(id, name)).WithExactArguments().WillReturn(...);

Hope that helps.

Answer 2

Disclaimer, I work in Typemock.

To avoid exceptions you should fake all future instatnces of MyDataContext:

var fakeDC = Isolate.Fake.AllInstances<MyDataContext>();

And then set the behavior fo MyFunction():

int? id = 10;
string name = "David";

Isolate.WhenCalled(() => fakeDC.MyFunction(id, name)).WithExactArguments().WillReturn(..);

It makes sure that all instances of MyDataContext will be faked (created by new MyDataContext() in any part of your program), and MyFunction() behavior will be faked as well.

Since i don't know all the details, check the example below for more understanding:

internal class Foo
{
    public Foo()
    {
    }

    public int Bar()
    {
        var x = new MyDataContext();
        return x.MyFunction(null, "5");
    }
}

public class MyDataContext : DataContext
{
   //
    public int MyFunction(int? a, string b)
    {
        if(a == null)
        {
            throw new Exception();
        }

        return 0;
    }
}

[TestMethod, Isolated]
public void TestMyDataContext()
{
     //Arrange
     var fakeDC = Isolate.Fake.AllInstances<MyDataContext>();
     Isolate.WhenCalled(() => fakeDC.MyFunction(null, "5")).WithExactArguments().WillReturn(6);

     //Act
     var foo = new Foo();
     var res = foo.Bar();

     //Assert
     Assert.AreEqual(6, res);
 }

See all information in our docs.