Expression Context

ExpressionContext stores runtime data for an expression evaluation: parameters, functions, async callbacks, and event handlers.

Use configuration for parser and evaluation settings.

Creating a context

var context = new ExpressionContext
{
    Parameters =
    {
        ["Price"] = 12.5m,
        ["Quantity"] = 4
    },
    Functions =
    {
        ["SecretOperation"] = _ => 42
    }
};

var expression = new Expression("Price * Quantity + SecretOperation()", context);
var result = expression.Evaluate();

To use configuration and context together, pass both to the expression:

var configuration = new ExpressionConfiguration
{
    Evaluation = new ExpressionEvaluationOptions
    {
        IgnoreCaseAtBuiltInFunctions = true
    }
};

var context = new ExpressionContext
{
    Parameters =
    {
        ["Value"] = -42
    }
};

var expression = new Expression("ABS(Value)", configuration, context);
var result = expression.Evaluate();

Lifetime

ExpressionContext is mutable and not thread-safe. Do not cache a single context as global shared state because parameters, functions, or handlers can change.

Create a context per evaluation, request, or operation:

public class EvaluationService(ExpressionConfiguration configuration)
{
    public object? Evaluate(string expressionText, IDictionary<string, object?> parameters)
    {
        var context = new ExpressionContext(parameters);
        var expression = new Expression(expressionText, configuration, context);
    
        return expression.Evaluate();
    }
}