C# Eval Expression Variables

The Z.Expressions.Eval library provides the following standard constants, which you can use directly in the expressions or script.

You can also define your custom variables and specify them as a parameter to use in the expression in any of the following ways.

• Anonymous Type
• Argument Position
• Class Member
• Dictionary

Anonymous Type

You can specify variables that you want to use in the expression as an anonymous type. The following example adds two variables a and b and used them inside an expression using anonymous type.

public static void Example1()
{
    string expression = "a*2 + b*3 - 3";
    int result = Eval.Execute<int>(expression, new { a = 10, b = 5 });
    Console.WriteLine("{0} = {1}", expression, result);
}

Argument Position

You can also specify values directly as argument position parameters as shown below.

public static void Example2()
{
    string expression = "{0}*2 + {1}*3 - 3";
    int result = Eval.Execute<int>(expression, 10, 5);
    Console.WriteLine("{0} = {1}", expression, result);
}

Class Member

The Z.Expressions.Eval library allows you to use class members inside expression by passing the class object as a parameter.

public static void Example3()
{
    string expression = "a*2 + b*3 - 3";
    dynamic expandoObject = new ExpandoObject();
    expandoObject.a = 10;
    expandoObject.b = 5;

    int result = Eval.Execute<int>(expression, expandoObject);
    Console.WriteLine("{0} = {1}", expression, result);
}

Dictionary

You can also define your custom variables in a dictionary and pass the object as a parameter

public static void Example4()
{
    string expression = "a*2 + b*3 - 3";

    var values = new Dictionary<string, object>() 
    { 
        { "a", 10 }, 
        { "b", 5 } 
    };

    int result = Eval.Execute<int>(expression, values);
    Console.WriteLine("{0} = {1}", expression, result);
}