C# in Depth

240 CHAPTER 9 Lambda expressions and expression treesRunning listing 9.6 will produce the output“(2 + 3),” which demonstrates thatthe various expression classes overrideToString to produce human-readableoutput. Figure 9.1 depicts the tree generatedby the code.It’s worth noting that the “leaf” expressionsare created first in the code: youbuild expressions from the bottom up.This is enforced by the fact that expressionsare immutable—once you’ve createdan expression, it will never change,so you can cache and reuse expressionsat will.Now that we’ve built up an expressiontree, let’s try to actually execute it.LeftfirstArgConstantExpressionNodeType=ConstantType=System.Int32Value=2addBinaryExpressionNodeType=AddType=System.Int32RightsecondArgConstantExpressionNodeType=ConstantType=System.Int32Value=3Figure 9.1 Graphical representation of theexpression tree created by listing 9.69.3.2 Compiling expression trees into delegatesOne of the types derived from Expression is LambdaExpression. The generic classExpression then derives from LambdaExpression. It’s all slightly confusing—figure9.2 shows the type hierarchy to make things clearer.The difference between Expression and Expression is that thegeneric class is statically typed to indicate what kind of expression it is, in terms ofreturn type and parameters. Fairly obviously, this is expressed by the TDelegate typeparameter, which must be a delegate type. For instance, our simple addition expressionis one that takes no parameters and returns an integer—this is matched by thesignature of Func, so we could use an Expression to represent theexpression in a statically typed manner. We do this using the Expression.Lambdamethod. This has a number of overloads—our examples use the generic method,which uses a type parameter to indicate the type of delegate we want to represent. SeeMSDN for alternatives.ExpressionLambdaExpressionBinaryExpression(Other types)ExpressionFigure 9.2Type hierarchy from Expression up to ExpressionLicensed to Rhona Hadida