Inspecting expression body

Learn to read Expression Body, NodeType and Parameter.

Inspecting the expression body

You traverse an expression tree with Body, NodeType, Parameters and Type. NodeType gives the operation kind (e.g. Add, Multiply, Lambda).

Body holds the inner expression. For x => x * x, Body is a Multiply node; Body.NodeType returns Multiply. This is how ORMs understand the query.

Reading parameters

expr.Parameters is the list of lambda parameters. Each element is a ParameterExpression with Name and Type properties. The lambda (a, b) => a + b contains two parameters.

Reading parameters is essential when reconstructing an expression tree or translating it to another form (e.g. SQL). Providers use this list to map each parameter to a column.

Exercise Inspect the expression body

Define a two-parameter expression tree with Expression<Func<int, int, int>> add = (a, b) => a + b;. Print add.Body.NodeType and add.Body separately with Console.WriteLine.

Show example solution

Define the expression tree with Expression<Func<int, int, int>> add = (a, b) => a + b;. Then call Console.WriteLine(add.Body.NodeType); and Console.WriteLine(add.Body);.
Example solution:
using System;
using System.Linq.Expressions;

class Program
{
    static void Main()
    {
        Expression<Func<int, int, int>> add = (a, b) => a + b;
        Console.WriteLine(add.Body.NodeType);
        Console.WriteLine(add.Body);
    }
}

Test yourself

Match NodeType value to its meaning

Lambda

Lambda expression
Multiply

Multiplication
Add

Addition
Constant

Constant value

Get the first parameter name

var name = expr.____[0].Name;

Show answer

var name = expr.Parameters[0].Name;

What does NodeType return?
- The parameter list of the lambda
- The operation kind of the expression (Multiply, Add, ...)
- The result value
- The string form of the expression
Show answer

The operation kind of the expression (Multiply, Add, ...) — Correct. NodeType returns the operation kind of the expression.

Next lesson: Expression.Compile

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