How Can I Efficiently Compare Lambda Expressions for Equality?

Most Efficient Way to Test Equality of Lambda Expressions

Problem

Consider the method signature:

public bool AreTheSame<T>(Expression<Func<T, object>> exp1, Expression<Func<T, object>> exp2)

How can you efficiently determine if two lambda expressions are equal? This method should only handle simple expressions, specifically MemberExpressions in the form c => c.ID.

Answer

public static bool Eq<TSource, TValue>(
    Expression<Func<TSource, TValue>> x,
    Expression<Func<TSource, TValue>> y)
{
    return ExpressionsEqual(x, y, null, null);
}

This method compares the expressions by breaking them down into their constituent parts and comparing each part individually. It does not perform a full AST comparison, but instead collapses any constant expressions and compares their values directly.

Implementation Details

The implementation of the ExpressionsEqual method recursively compares the different parts of the expressions. For example, if the expressions are MemberExpressions, it compares the Member property of each expression. If they are BinaryExpressions, it compares the Method, Left, and Right properties.

The method handles special cases like ConditionalExpressions and NewArrayExpressions. It also handles cases where the expressions have different types, including anonymous types.

Performance

The performance of this method is significantly better than a full AST comparison, especially for expressions that contain constant values. This makes it suitable for use cases like mock validation, where the lambda expression may reference local variables that should be compared by value.

NuGet Package

The code is available as a NuGet package, which can be installed using the following command:

Install-Package LambdaCompare

Usage

The usage of the Eq method is straightforward:

var f1 = GetBasicExpr1();
var f2 = GetBasicExpr2();
Assert.IsTrue(LambdaCompare.Eq(f1, f2));

Conclusion

This efficient method for comparing lambda expressions provides a convenient and performant way to determine their equality. It is particularly useful in scenarios where local variable values need to be compared.

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