Flee Variables

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

You can also define your custom variables by using the Variables property of ExpressionContext.

• The VariableCollection object that is returned is a dictionary of strings to objects.
• The first character of a variable name must be a letter or underscore, and the following characters can be letters, underscores, or numbers.

The following example adds two variables x and y, and used them inside an expression.

public static void Example1()
{
    ExpressionContext context = new ExpressionContext();

    context.Variables["x"] = 6;
    context.Variables["y"] = 5.5;

    IDynamicExpression e = context.CompileDynamic("x + y * (x - y)");
}

Flee is statically typed, it knows the type of each variable based on the value.

public static void Example2()
{
    ExpressionContext context = new ExpressionContext();
    context.Variables["myInt"] = 6;
    context.Variables["myDouble"] = -3.6;
    context.Variables["myStr"] = "Hello World";
    context.Variables["myArray"] = new object[] { 3.5, "Test", false };

    List<string> expressions = new List<string>()
    {
        "myInt + myDouble",
        "myStr + \", you are learning Expression Evaluator\"",
        "myArray.Length",
        "myArray[0]",
        "myArray[2]",
    };

    foreach (var expression in expressions)
    {
        IDynamicExpression eDynamic = context.CompileDynamic(expression);

        Object result = eDynamic.Evaluate();
        Console.WriteLine("{0} = {1}", expression, result);
    }
}

Let's execute the above code, and you will see the following output.

myInt + myDouble = 2.4
myStr + ", you are learning Expression Evaluator" = Hello World, you are learning Expression Evaluator
myArray.Length = 3
myArray[0] = 3.5
myArray[2] = False

You can also use expressions as variables in other expressions. When the top-level expression is evaluated, each sub-expression will be evaluated to provide the variable's value.

public static void Example3()
{
    ExpressionContext context = new ExpressionContext();
    context.Variables.Add("a", 64);
    IDynamicExpression e1 = context.CompileDynamic("a / 8");

    context = new ExpressionContext();
    context.Variables.Add("a", 3);
    IDynamicExpression e2 = context.CompileDynamic("a * a");

    context = new ExpressionContext();
    context.Variables.Add("x", e1);
    context.Variables.Add("y", e2);

    IDynamicExpression eDynamic = context.CompileDynamic("x + y");
    Object result = eDynamic.Evaluate();
    Console.WriteLine(result);
}