Custom Syntax

The C# Eval Expression supports most C# keywords and operators.

On top of that, it introduces additional syntax to make your expressions more flexible, more concise, and easier to write.

These features allow you to go beyond standard C# by simplifying common patterns and unlocking new possibilities.

In this section, you will discover custom expressions, keywords, and operators designed to improve your productivity.

Expression

Constant DateTime

Allow you to specify a DateTime value from a string representation.

The value must be wrapped with # to be recognized as a DateTime.

DateTime result = Eval.Execute<DateTime>("#1981/04/13 10:10#");

Online Example

You can use any valid date and time format supported by DateTime.

Constant TimeSpan

Allow you to specify a TimeSpan value from a string representation.

The value must be wrapped with ## to be recognized as a TimeSpan.

TimeSpan result = Eval.Execute<TimeSpan>("##10:10##");

Online Example

You can use any valid time format supported by TimeSpan.

Inner Expression

Allow you to use inner expressions inside another expression.

Inner expressions are wrapped with { ... } and are evaluated before the outer expression. You can use return, but it’s optional when the expression contains a single statement.

var result = Eval.Execute("return 1 + { return 2 * 3 } + { 5 * 6 }");

// Result: 1 + 6 + 30 = 37

Online Example

Recursive Lambda Expression

Allow you to create a recursive lambda expression.

You can reference the lambda inside its own body to perform recursive operations.

var result = Eval.Execute<int>(@"
	Func<int, int> fib = fib(x) => { 
		return x > 2 ? fib(x - 1) + fib(x - 2) : 1; 
	};
	return fib(X);
", new { X = 5 });

// Result: 5

Online Example

Keywords

Break All

Break all loops (do, for, foreach, while) or a switch statement.

var result = Eval.Execute<int>(@"
int result = 0;

for (int x1 = 0; x1 < 10; x1++)
{
    result += x1;

    for (int x2 = 0; x2 < 10; x2++)
    {
        result += x2;

        for (int x3 = 0; x3 < 10; x3++)
        {
            result += x3;
            break all; // break all loops
        }
    }
}

return result;		
");

// Result: 0 + 0 + 0 = 0

Online Example

This keyword is very useful in a switch statement to break the outer loop or the switch itself.

Break Outer

Break the current loop and its immediate parent loop (do, for, foreach, while) or switch statement.

var result = Eval.Execute<int>(@"
int result = 0;

for (int x1 = 0; x1 < 10; x1++)
{
    result += x1;

    for (int x2 = 0; x2 < 10; x2++)
    {
        result += x2;

        for (int x3 = 0; x3 < 10; x3++)
        {
            result += x3;
            break outer; // break current loop and its immediate parent loop 
        }
    }
}

return result;		
");

// Result: 0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45 (all value coming from the `result += x1;`

Online Example

This keyword is very useful in a switch statement to break the outer loop or the switch itself.

Operators

Array Implicit ..

Declare an implicit range that can be used in a collection initializer.

var result = Eval.Execute("var list = new List<int>() { 1, 2..100 };");

Online Example

The expression 2..100 automatically expands to all values from 2 to 100.

Exclusive-OR - Alternative ^|

The exclusive-OR operator alternative. Works the same as the ^ operator.

var result = Eval.Execute("true ^| false");

// Result: true

Online Example

Use ^| and ^|= when the ^ operator is configured for another purpose.

This alternative syntax is useful when the option UseCaretForExponent = true.

Exclusive-OR Assignation - Alternative ^|=

The exclusive-OR assignment operator alternative. Works the same as the ^= operator.

var result = Eval.Execute(@"
	bool b = true;
	b ^|= false;
	return b;
");

// Result: true

Online Example

Use ^|= when the ^= operator is configured for another purpose.

This alternative syntax is useful when the option UseCaretForExponent = true.

Exponent ^^

Raise a number to the power of another number.

The result is always a double.

var result = Eval.Execute("2^^3");

// Result: 8

Online Example

Use ^^ to avoid conflicts when the ^ operator is already used for exclusive-OR.

Alternatively, you can use the option UseCaretForExponent = true.

Exponent Assignment ^^=

Raise the value of a variable or property to the power of an expression, and assign the result back to the variable or property.

The result is always a double.

var result = Eval.Execute(@"
	var x = 2;
	x ^^= 3;
	return x;
");

// Result: 8

Online Example

Use ^^= to avoid conflicts when the ^= operator is configured for another purpose.

Alternatively, you can use the option UseCaretForExponent = true.

Not Equals - Alternative <>

The inequality operator alternative. Works the same as !=.

Returns false if the operands are equal; otherwise, returns true.

var result = Eval.Execute("2 + 2 <> 4");

// Result: false

Online Example

Summary

The C# Eval Expression extends standard C# by adding custom syntax for expressions, keywords, and operators.

These features help you write shorter, clearer, and more powerful expressions without changing how C# works.

  • Use custom expressions to simplify complex logic
  • Use additional keywords to control execution flow more easily
  • Use alternative operators to avoid conflicts and improve readability

All these additions are designed to improve your productivity while keeping a familiar C# experience.

Last updated: 2026-05-04
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