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    <p> The code that throws the exception is</p> <pre class="lang-csharp prettyprint-override"><code>typeof(Expression).GetMethod("Lambda") </code></pre> <p>because there are 18 methods called <code>Lambda</code> defined on the <code>Expression</code> type (thus the <code>AmbiguousMatchException</code>).</p> <p><code>GetMethod(string methodName)</code> is appropriate when there are no overloads. In this case I would use <code>GetMethods()</code> and then filter out the one I need.</p> <p>In your case, the right overload is</p> <pre class="lang-csharp prettyprint-override"><code>Expression.Lambda&lt;TDelegate&gt;(Expression body, params ParameterExpression[] parameters) </code></pre> <p>You could write a function that validates the right overload by checking the number of parameters and their type, but I found an easier alternative: filter the method by the <code>.ToString()</code> representation, which in our case is:</p> <pre class="lang-csharp prettyprint-override"><code>System.Linq.Expressions.Expression`1[TDelegate] Lambda[TDelegate](System.Linq.Expressions.Expression, System.Linq.Expressions.ParameterExpression[]) </code></pre> <p>There's also a problem with the way you pass the parameters (<code>new object[] { expression, param }</code>). The second parameter is not of type <code>ParameterExpression</code>, but <code>ParameterExpression[]</code> (array), therefore you should pass <code>new[]{param}</code> instead of just <code>param</code>. When calling it in regular code it works like that because it's defined as <code>params ParameterExpression[]</code>.</p> <p>In conclusion, the following code should work in your case:</p> <pre class="lang-csharp prettyprint-override"><code>const string methodSignature = "System.Linq.Expressions.Expression`1[TDelegate] Lambda[TDelegate]" + "(System.Linq.Expressions.Expression, System.Linq.Expressions.ParameterExpression[])"; var lambdaMethod = typeof (Expression).GetMethods() .Single(mi =&gt; mi.ToString() == methodSignature); var funcType = typeof (Func&lt;,&gt;).MakeGenericType(objectType, typeof (object)); var genericLambda = lambdaMethod.MakeGenericMethod(funcType); var selectExpression = genericLambda.Invoke(null, new object[] { expression, new[] { param } }); </code></pre>
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    1. COActually [there are 18 methods called `Lambda` on `Expression`](http://msdn.microsoft.com/en-us/library/system.linq.expressions.expression.lambda.aspx) and 6 of them are generic. So, your code will return one of the generic ones, but I think it most likely won't be the right one.
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    2. COThanks.. Now I get: A object of the type 'System.Linq.Expressions.TypedParameterExpression' can’t be converted to the type 'System.Linq.Expressions.ParameterExpression[]'. At the Invoke method.
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    3. CO@svick: How do I select the right one then?
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      1. USMorten Holmgaard
 

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