Question

Is it possible to write code like the following. I’m trying to using Moq with objects that I’m reflecting on as part of a testing framework. The code below raises a "Unhandled expression type: ‘Goto’" exception from Moq, which I guess is expecting something different. It kind of looks like it should work though!

    private void button1_Click(object sender, EventArgs e)
    {
        Ifoo  = foo Foo();

        // Create input parameter for lambda
        ParameterExpression value = Expression.Parameter(typeof(IFoo), "value");

        // create return statement for lambda
        Expression setupProperty = Expression.Return(Expression.Label(), Expression.Property(value, "Bar"), typeof(string));

        // convert expression to lambda (should now be the equivalent of "v => v.Bar")
        var func = Expression.Lambda<Func<IFoo, string>>(setupProperty, value);//.Compile();
        //string s = func(foo); // this bit works fine if .Compile() is included

        var mockFoo = new Mock<IFoo>();

        mockFoo.SetupProperty(func); // exception thrown by moq here, obviously isn't exactly the same as "v => v.Bar"
        mockFoo.Object.Bar = "Burge+";
    }

Thanks!

Answer 1

Ok, this is possible, here is the corrected code.

// Create input parameter for lambda 
ParameterExpression value = Expression.Parameter(typeof(IFoo), "value"); 

// create return statement for lambda 
Expression setupProperty = Expression.Property(value, "Bar"); 

// convert expression to lambda (should now be the equivalent of "v => v.Bar") 
var func = Expression.Lambda<Func<IFoo, string>>(setupProperty, value);

var mockFoo = new Mock<IFoo>(); 
mockFoo.SetupProperty(func); // this works now
mockFoo.Object.Bar = "Burge+"; 

I investigated this by creating an expression from a lambda using the code below

Expression<Func<IFoo, string>> setupBar = v => c.Bar;

I then looked at this in the debugger in vs 2010. Expressions have a “Debug View” that shows a text representation of the expression so it is possible to add a watch on that or something similar. The above comes out as

 .Lambda #Lambda1<System.Func`2[WindowsFormsApplication1.IFoo,System.String]>(WindowsFormsApplication1.IFoo
 $v) {
   $v.Bar
 }

I looked at this and tried to work out what Expressions would make this, then created an expression and compared it in the debugger.

The interesting thing for me is that although this expression returns a value there is no assignment or return statement. I guess this must be implicit somehow.