Building Expression Evaluator with Expression Trees in C# – Improving the Api

This post is part of a series about building a simple mathematical expression evaluator. For previous posts and full source code see Table of Contents.

Introduction

In previous part we added support for variables but it has a big disadvantage: values of the variables are bound by position and not by name. This means that you should pass values for the variables in the same order as they appear in the expression. For example the following test will fail:

public void Can_Process_Multiple_Variables()

{

    var a = 6;

    var b = 4.32m;

    var c = 24.15m;

    Assert.That(engine.Evaluate("(c+b)*a", a, b, c), Is.EqualTo((c + b) * a));

}

On the other hand binding by name has following advantages: If you change the expression you do not have to worry about keeping order of the variables you pass in sync. So in this post we are going to change expression evaluator to bind parameters by name. We will also add a new overload for better performance when we need to evaluate same expression with different parameter values.

Binding Variables by Name

The easiest way to pass variables with their name is to pass a dictionary of variables with variable name acting as a key. While this will work it is not the most elegant and readable solution. A better solution will be to pass an anonymous object with properties pointing to the variable values. So our first task is to make the following test pass:

public void Can_Process_Multiple_Variables()

{

    var a = 6;

    var b = 4.32m;

    var c = 24.15m;

    Assert.That(engine.Evaluate("(c+b)*a", new { a, b, c }), Is.EqualTo((c + b) * a));

}

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