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();

Copying a context

Create a context from an existing context when an evaluation needs the same parameters, functions, and event handlers without sharing its mutable dictionaries:

var original = new ExpressionContext
{
    Parameters =
    {
        ["Price"] = 12.5m,
        ["Quantity"] = 4
    }
};

var copy = new ExpressionContext(original);
copy.Parameters["Quantity"] = 10;

The constructor creates new dictionaries for parameters and functions, so adding, updating, or removing entries in copy does not modify original. Parameter values, function delegates, and event handlers are copied by reference.

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();
    }
}