Support for debugging lambda expressions with Visual Studio 2015

Anyone who uses LINQ (or lambdas in general) and the debugger will quickly discover the dreaded message “Expression cannot contain lambda expressions”. Lack of lambda support has been a limitation of the Visual Studio Debugger ever since Lambdas were added to C# and Visual Basic.  With visual studio 2015 Microsoft has added support for debugging lambda expressions.

Let’s first look at an example, and then I’ll walk you through current limitations.

Example

To try this yourself, create a new C# Console app with this code:

using System.Diagnostics;
using System.Linq;

class Program
{
    static void Main()
    {
        float[] values = Enumerable.Range(0, 100).Select(i => (float)i / 10).ToArray();

        Debugger.Break();
    }
}

Then compile, start debugging, and add “values.Where(v => (int)v == 3).ToArray()” in the Watch window. You’ll be happy to see the same as what the screenshot above shows you. I am using Visual Studio 2015 Preview and it has some limitations.

Known Limitations with the New Expression Evaluators at Preview

• Make Object ID is not implemented
• ASP.NET 5 with 64-bit IIS and remote debugging ASP.NET 5 is not supported. The debugger will automatically use the old expression evaluator in these cases.
• Calling LINQ methods doesn’t work if System.Core.dll hasn’t been loaded in the process yet.
  The error message you see will be similar to “error CS1061: 'int[]' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'int[]' could be found (are you missing a using directive or an assembly reference?)”
  Executing a System.Core.dll method in your app will force it to load. (Enumerable. Range for example)
• Declaring variables in the Immediate window isn’t supported
• Evaluation inside async and iterator methods is unreliable
• Dynamic variables can’t be inspected
• Evaluation of/in dynamic types generated using Reflection. Emit doesn’t work.
• Support for evaluation of COM objects is limited.

Working Around Limitations of the New Expression Evaluators

If you run into any issues that make the new Expression Evaluator unusable or you need a feature that we haven’t implemented yet, it’s still possible to use the old expression evaluator. To switch back to the old expression evaluator, go to Debug -> Options -> General, and check the box “Use the legacy C# and VB expression evaluators”. Of course, at that point, you will not have lambda support in the debugger at all.

If you do run into problems not listed above that require you to use the legacy expression evaluators, please let us know so we can address them.