C# Statement Lambda

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Syntax

(input_parameters) => { sequence_of_statements; }

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Technical Characteristics

• Delegate Resolution: A statement lambda must be convertible to a compatible delegate type, typically from the Action family (for void returns) or the Func family (for value returns).
• Return Semantics: If the target delegate type has a non-void return type, the statement block must contain an explicit return statement on all reachable code paths.
• Expression Tree Incompatibility: Unlike expression lambdas, statement lambdas cannot be converted into expression trees (System.Linq.Expressions.Expression<TDelegate>). The C# compiler can only emit them as executable Intermediate Language (IL) bound to a delegate instance.
• Lexical Scoping and Closures: The statement block defines a local scope. It can capture variables from the enclosing method (forming a closure), but variables declared within the { } block are strictly local to the lambda’s invocation.
• Asynchronous Execution: Statement lambdas can be modified with the async keyword, allowing the use of await within the statement block. This resolves to an Action (if returning void), Func<Task>, or Func<Task<T>>.

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Syntax Visualization

Action<string> processString = (input) => 
{
    string normalized = input.Trim().ToLower();
    Console.WriteLine(normalized);
};

Func<int, int, int> computeWithBranching = (x, y) => 
{
    int sum = x + y;
    if (sum < 0) 
    {
        return 0; // Explicit return required
    }
    return sum;   // Explicit return required
};

Func<string, Task<int>> fetchAndMeasureAsync = async (url) => 
{
    using var client = new HttpClient();
    string result = await client.GetStringAsync(url);
    return result.Length;
};

// Using discards (_) for unused parameters in the delegate signature
Func<int, int, string> ignoreParameters = (_, _) => 
{
    string defaultState = "Unchanged";
    return defaultState;
};