How to Convert a .NET Func to an Expression?

Converting a .NET Func to a .NET Expression>

In .NET, a lambda expression can be easily converted into an Expression<> using the Expression class. However, there may be instances where one wishes to convert a Func back into an Expression>.

Consider the following code:

public void GimmeExpression(Expression<Func<T>> expression)
{
    ((MemberExpression)expression.Body).Member.Name; // "DoStuff"
}

public void SomewhereElse()
{
    GimmeExpression(() => thing.DoStuff());
}

Here, a lambda expression is passed to the GimmeExpression method, which extracts the name of the method being called (DoStuff).

Now, suppose we want to perform a similar operation but in a restricted context:

public void ContainTheDanger(Func<T> dangerousCall)
{
    try 
    {
        dangerousCall();
    }
    catch (Exception e)
    {
        // This next line does not work...
        Expression<Func<T>> DangerousExpression = dangerousCall;
        var nameOfDanger = 
            ((MemberExpression)dangerousCall.Body).Member.Name;
        throw new DangerContainer(
            "Danger manifested while " + nameOfDanger, e);
    }
}

public void SomewhereElse()
{
    ContainTheDanger(() => thing.CrossTheStreams());
}

In this case, we attempt to convert the dangerousCall Func into an Expression>, but we encounter a compile-time error:

Cannot implicitly convert type 'System.Func<T>' to 'System.Linq.Expressions.Expression<System.Func<T>>'. An explicit cast does not resolve the situation.

This is because Func represents a generic delegate rather than an expression. While the compiler can magical convert lambdas to Expression<> objects, this conversion is not possible for general Func objects.

Disassembling the intermediate language (IL) and inferring the original expression might allow for this conversion if the necessary data has not been optimized away. However, this is a complex and potentially unreliable approach.

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