Figure 124: Object Graph of Unary and Binary Expressions in C#.NET

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Figure 124: Object Graph of Unary and Binary Expressions
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12: Delegates and Lambda Expressions Lambda Expressions versus Expression Trees
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Both a lambda expression for delegates and a lambda expression for an expression tree are compiled, and in both cases the syntax of the expression is verified at compile time with full semantic analysis The difference, however, is that a lambda expression is compiled into a delegate in CIL In contrast, an expression tree is compiled into a data structure of type SystemLinqExpressionsExpression Let us consider an example that highlights the difference between a delegate and an expression tree SystemLinqEnumerable and SystemLinqQueryable are very similar They each provide virtually identical extension methods to the collection interfaces they extend (IEnumerable and IQueryable, respectively) Consider, for example, the Where() method from Listing 1223 Given a collection that supports IEnumerable, a call to Where() could be as follows:
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personsWhere( person => personNameToUpper() == "INIGO MONTOYA");
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Conceptually, the Enumerable extension method signature is defined on IEnumerable<TSource> as follows:
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public IEnumerable<TSource> Where<TSource>( Func<TSource, bool> predicate);
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However, the equivalent Queryable extension on the IQueryable<TSource> method call is identical, even though the conceptual Where() method signature (shown) is not:
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public IQueryable<TSource> Where<TSource>( Expression<Func<TSource, bool>> predicate);
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The calling code for the argument is identical because the lambda expression itself does not have type until it is assigned/cast Enumerable s Where() implementation takes the lambda expression and converts it to a delegate that the Where() method s implementation calls In contrast, when calling Queryable s Where(), the lambda expression is converted to an expression tree so that the compiler converts the lambda expression into data The object implementing IQueryable receives
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the expression data and manipulates it As suggested before, the expression tree received by Where() may be converted into a SQL query that is passed to a database
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Examining an Expression Tree
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Capitalizing on the fact that lambda expressions don t have intrinsic type, assigning a lambda expression to a SystemLinqExpressionsExpression<TDelegate> creates an expression tree rather than a delegate In Listing 1224, we create an expression tree for the Func<int, int, bool> (Recall that Func<int, int, bool> is functionally equivalent to the ComparisonHandler delegate) Notice that just the simple act of writing an expression to the console, ConsoleWriteLine(expression) (where expression is of type Expression<TDelegate>), will result in a call to expression s ToString() method However, this doesn t cause the expression to be evaluated or even to write out the fully qualified name of Func<int, int, bool> (as would happen if we used a delegate instance) Rather, displaying the expression writes out the data (in this case, the expression code) corresponding to the value of the expression tree
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Listing 1224: Examining an Expression Tree
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using System; using SystemLinqExpressions; class Program { static void Main() { Expression<Func<int, int, bool>> expression; expression = (x, y) => x > y; ConsoleWriteLine("-------------{0}-------------", expression); PrintNode(expressionBody, 0); ConsoleWriteLine(); ConsoleWriteLine(); expression = (x, y) => x * y > x + y; ConsoleWriteLine("-------------{0}-------------", expression); PrintNode(expressionBody, 0); ConsoleWriteLine(); ConsoleWriteLine(); }
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12: Delegates and Lambda Expressions
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public static void PrintNode(Expression expression, int indent) { if (expression is BinaryExpression) PrintNode(expression as BinaryExpression, indent); else PrintSingle(expression, indent); } private static void PrintNode(BinaryExpression expression, int indent) { PrintNode(expressionLeft, indent + 1); PrintSingle(expression, indent); PrintNode(expressionRight, indent + 1); } private static void PrintSingle( Expression expression, int indent) { ConsoleWriteLine("{0," + indent * 5 + "}{1}", "", NodeToString(expression)); } private static string NodeToString(Expression expression) { switch (expressionNodeType) { case ExpressionTypeMultiply: return "*"; case ExpressionTypeAdd: return "+"; case ExpressionTypeDivide: return "/"; case ExpressionTypeSubtract: return "-"; case ExpressionTypeGreaterThan: return ">"; case ExpressionTypeLessThan: return "<"; default: return expressionToString() + " (" + expressionNodeTypeToString() + ")"; } } }
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In Output 123, we see that the ConsoleWriteLine() statements within Main() print out the body of the expression trees as text
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OUTPUT 123:
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------------- (x, y) => x > y ------------x (Parameter) > y (Parameter)
------------- (x, y) => (x * y) > (x + y) ------------x (Parameter) * y (Parameter) > x (Parameter) + y (Parameter)
The output of the expression as text is due to conversion from the underlying data of an expression tree conversion similar to the PrintNode() and NodeTypeToString() functions, only more comprehensive The important point to note is that an expression tree is a collection of data, and by iterating over the data, it is possible to convert the data to another format In the PrintNode() method, Listing 1224 converts the data to a horizontal text interpretation of the data However, the interpretation could be virtually anything Using recursion, the PrintNode() function demonstrates that an expression tree is a tree of zero or more expression trees The contained expression trees are stored in an Expression s Body property In addition, the expression tree includes an ExpressionType property called NodeType where ExpressionType is an enum for each different type of expression There are numerous types of expressions: BinaryExpression, ConditionalExpression, LambdaExpression (the root of an expression tree), MethodCallExpression, ParameterExpression, and ConstantExpression are examples Each type derives from SystemLinqExpressionsExpression Generally, you can use statement lambdas interchangeably with expression lambdas However, you cannot convert statement lambdas into expression trees You can express expression trees only by using expression lambda syntax