{"id":2833,"date":"2009-11-19T12:05:00","date_gmt":"2009-11-19T12:05:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/visualstudio\/2009\/11\/19\/debugging-expression-trees-in-visual-studio-2010\/"},"modified":"2019-02-14T15:42:07","modified_gmt":"2019-02-14T23:42:07","slug":"debugging-expression-trees-in-visual-studio-2010","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/visualstudio\/debugging-expression-trees-in-visual-studio-2010\/","title":{"rendered":"Debugging Expression Trees in Visual Studio 2010"},"content":{"rendered":"

First of all, let\u2019s take a look at the example from one of my previous posts<\/a>. It creates an expression tree for calculating the factorial of a number.<\/p>\n

ParameterExpression <\/span>value =\n    Expression<\/span>.Parameter(typeof<\/span>(int<\/span>), \"value\"<\/span>);\nParameterExpression <\/span>result =\n    Expression<\/span>.Parameter(typeof<\/span>(int<\/span>), \"result\"<\/span>);\nLabelTarget <\/span>label = Expression<\/span>.Label(typeof<\/span>(int<\/span>));\nBlockExpression <\/span>block = Expression<\/span>.Block(\n    new<\/span>[] { result },\n    Expression<\/span>.Assign(result, Expression<\/span>.Constant(1<\/span>)),\n    Expression<\/span>.Loop(\n        Expression<\/span>.IfThenElse(\n            Expression<\/span>.GreaterThan(value,\n                Expression<\/span>.Constant(1<\/span>)),\n            Expression<\/span>.MultiplyAssign(result,\n                Expression<\/span>.PostDecrementAssign(value)),\n        Expression<\/span>.Break(label, result)\n        ),\n        label\n    )\n);\nExpression<\/span><Func<\/span><int<\/span>, int<\/span>>> lambda =\n    Expression<\/span>.Lambda<Func<\/span><int<\/span>, int<\/span>>>(block, value);\nConsole<\/span>.WriteLine(lambda.Compile()(5<\/span>));<\/pre>\n
<\/a>\nNow, what if you want to see the content of this tree in debug mode? You can try lambda.ToString()<\/span> but it\u2019s not very informative. All you get is this: value => {var result; \u2026 }<\/span>. Basically, it tells you only that the lambda has the result<\/span> parameter.\nAnother option is to explore the tree structure in the Watch<\/b> window. However, since this is a tree, you need to click through numerous nodes. This gives you a good understanding of the tree structure, but it might be hard to understand the actual content of the tree.\nBut there is a better solution: expression trees now have their own visualizer that helps you to explore their structure and content.\nCopy the above code example into Visual Studio 2010, set a breakpoint at the last line, and press F5. Rest the mouse pointer over the lambda variable and you will see that it has the DebugView<\/strong> property. This property is private and is exposed only in the debugger, so you can\u2019t use it in your code. But it\u2019s used to hook up the expression trees visualizer. Click the magnifying glass icon and you will see the list of available visualizers.\n\"image\"<\/a>\n<\/a><\/a>\nNow click Text Visualizer<\/b> and you get this.\n\"image\"<\/a>\nAs you can see, the content of the expression tree is represented in some kind of metalanguage. (No, we couldn’t use C# here, because expression trees serve other languages as well.)\nThere are some simple rules used in this metalanguage:<\/p>\n