DollarSignEngine

Dynamically evaluate and interpolate C# expressions at runtime with ease, leveraging the power of the Roslyn compiler.

Introduction

The DollarSignEngine is a robust C# library designed to simplify the process of dynamically evaluating and interpolating expressions at runtime. Ideal for applications requiring on-the-fly evaluation of string templates, it offers developers the flexibility to inject variables and execute complex C# expressions with the same syntax as C# string interpolation.

Guiding Principles

1. Core Purpose: Enable runtime evaluation of C# string interpolation ($"{}") exactly as it works in compile-time C#.

2. Extension Rules:

   • All features natively supported in C# string interpolation must be maintained
   • Avoid extending functionality beyond standard C# interpolation syntax
   • Only add features essential for runtime evaluation

Features

• Dynamic Expression Evaluation: Evaluate C# expressions dynamically at runtime, with support for interpolation and complex logic.
• Flexible Parameter Injection: Easily pass parameters into expressions using dictionaries, anonymous objects, or regular C# objects.
• Method & LINQ Support: Call methods on objects and use LINQ expressions within interpolated strings.
• Format Specifiers & Alignment: Full support for C# format specifiers and alignment in interpolated expressions.
• Custom Variable Resolution: Provide custom variable resolvers for advanced use cases.
• Multiple Syntax Options: Support for both standard C# interpolation {expression} and dollar-sign ${expression} syntax.
• Comprehensive Error Handling: Detailed exceptions with helpful error messages and suggestions for common issues.
• Expression Caching: Compiled expressions are cached for improved performance with repeated evaluations.
• Security Validation: Built-in expression validation with configurable security levels.
• Multi-Framework Support: Compatible with .NET Standard 2.1, .NET 5, 6, 7, 8, and 9.
• Performance Monitoring: Built-in metrics for cache hit rates and evaluation performance.

Installation

The library is available on NuGet. You can install it using the following command:

dotnet add package DollarSignEngine

Usage

Below are some examples of how to use the DollarSignEngine to evaluate expressions dynamically.

Basic Interpolation

// Simple interpolation with current date var result = await DollarSign.EvalAsync("Today is {DateTime.Now:yyyy-MM-dd}"); Console.WriteLine(result); // Outputs: Today is 2023-05-01 (based on current date) // With parameters using anonymous object var name = "John"; var result = await DollarSign.EvalAsync("Hello, {name}!", new { name }); Console.WriteLine(result); // Outputs: Hello, John! // With parameters using dictionary var parameters = new Dictionary<string, object?> { { "name", "John" } }; var result = await DollarSign.EvalAsync("Hello, {name}!", parameters); Console.WriteLine(result); // Outputs: Hello, John!

Using Custom Objects

// Using a class directly public class User { public string Username { get; set; } = string.Empty; public int Age { get; set; } } var user = new User { Username = "Alice", Age = 30 }; var result = await DollarSign.EvalAsync("User: {Username}, Age: {Age}", user); Console.WriteLine(result); // Outputs: User: Alice, Age: 30 // Using anonymous types with nested properties var person = new { Name = "Jane", Address = new { City = "New York", Country = "USA" } }; var result = await DollarSign.EvalAsync("Person: {Name} from {Address.City}", person); Console.WriteLine(result); // Outputs: Person: Jane from New York // Calling a method on a custom object public class Greeter { public string Name { get; set; } = string.Empty; public string Hello() { return $"hello, {Name}"; } } var greeter = new Greeter { Name = "Bob" }; var options = new DollarSignOptions { SupportDollarSignSyntax = true }; var result = await DollarSign.EvalAsync("Greeting: ${Hello()}", greeter, options); Console.WriteLine(result); // Outputs: Greeting: hello, Bob // Without dollar sign syntax, the expression is treated as literal var standardResult = await DollarSign.EvalAsync("Greeting: ${Hello()}", greeter); Console.WriteLine(standardResult); // Outputs: Greeting: ${Hello()}

Method Calls and LINQ Expressions

// Calling methods on strings var text = "hello world"; var result = await DollarSign.EvalAsync("Uppercase: {text.ToUpper()}", new { text }); Console.WriteLine(result); // Outputs: Uppercase: HELLO WORLD // Using LINQ with collections var numbers = new List<int> { 1, 2, 3, 4, 5 }; var result = await DollarSign.EvalAsync("Even numbers: {numbers.Where(n => n % 2 == 0).Count()}", new { numbers }); Console.WriteLine(result); // Outputs: Even numbers: 2 // Chaining method calls var data = " sample text "; var result = await DollarSign.EvalAsync("Processed: {data.Trim().ToUpper().Replace('A', 'X')}", new { data }); Console.WriteLine(result); // Outputs: Processed: SXMPLE TEXT // Using strongly-typed custom objects for method calls public class Calculator { public int Add(int a, int b) => a + b; public int Multiply(int a, int b) => a * b; } var calc = new Calculator(); var result = await DollarSign.EvalAsync("Sum: {calc.Add(5, 3)}, Product: {calc.Multiply(5, 3)}", new { calc }); Console.WriteLine(result); // Outputs: Sum: 8, Product: 15

Important Note: For method calls and LINQ expressions to work properly, you need to pass strongly-typed objects rather than anonymous types. This is because the engine needs access to the type information at runtime to properly compile and execute the methods.

Format Specifiers and Alignment

// Using format specifiers var price = 123.456; var result = await DollarSign.EvalAsync("Price: {price:C2}", new { price }); Console.WriteLine(result); // Outputs: Price: $123.46 // Using alignment var number = 42; var result = await DollarSign.EvalAsync("Left aligned: {number,-10} | Right aligned: {number,10}", new { number }); Console.WriteLine(result); // Outputs: Left aligned: 42 | Right aligned: 42 // Combined alignment and format var percentage = 0.8654; var result = await DollarSign.EvalAsync("Progress: {percentage,8:P1}", new { percentage }); Console.WriteLine(result); // Outputs: Progress: 86.5%

Conditional Logic

// Simple ternary operation var age = 20; var result = await DollarSign.EvalAsync("You are {(age >= 18 ? \"adult\" : \"minor\")}.", new { age }); Console.WriteLine(result); // Outputs: You are adult. // Nested ternary operations var score = 85; var result = await DollarSign.EvalAsync("Grade: {(score >= 90 ? \"A\" : score >= 80 ? \"B\" : \"C\")}.", new { score }); Console.WriteLine(result); // Outputs: Grade: B. // Complex condition with formatting var price = 123.456; var discount = true; var result = await DollarSign.EvalAsync("Final price: {(discount ? price * 0.9 : price):C2}", new { price, discount }); Console.WriteLine(result); // Outputs: Final price: $111.11

Working with Collections

var numbers = new List<int> { 1, 2, 3, 4, 5 }; var result = await DollarSign.EvalAsync("Total sum: {numbers.Sum()}. Items count: {numbers.Count}", new { numbers }); Console.WriteLine(result); // Outputs: Total sum: 15. Items count: 5 var settings = new Dictionary<string, string> { { "Theme", "Dark" }, { "FontSize", "12" } }; var result = await DollarSign.EvalAsync("Theme: {settings[\"Theme\"]}, Font Size: {settings[\"FontSize\"]}", new { settings }); Console.WriteLine(result); // Outputs: Theme: Dark, Font Size: 12

Dollar Sign Syntax

// When working with text that contains literal curly braces (like JSON), // enable dollar sign syntax to specify which parts should be interpolated var options = new DollarSignOptions { SupportDollarSignSyntax = true }; var user = new { name = "Alice", age = 30 }; // With dollar sign syntax enabled, only ${...} is interpolated var jsonTemplate = "{ \"user\": { \"name\": \"{name}\", \"age\": ${age} } }"; var result = await DollarSign.EvalAsync(jsonTemplate, user, options); Console.WriteLine(result); // Outputs: { "user": { "name": "{name}", "age": 30 } } // In standard mode (default), all {...} expressions are interpolated var standardResult = await DollarSign.EvalAsync("{ \"user\": { \"name\": \"{name}\", \"age\": {age} } }", user); Console.WriteLine(standardResult); // Outputs: { "user": { "name": "Alice", "age": 30 } }

Method Calls and LINQ: Requirements and Limitations

When using method calls and LINQ expressions within interpolated strings, there are a few important considerations:

Requirements

1. Strongly-Typed Objects: For method calls to work properly, pass strongly-typed objects rather than anonymous types whenever possible. The engine needs access to type information at runtime.

   // This works well - using a concrete class var calculator = new Calculator(); var result = await DollarSign.EvalAsync("Result: {calculator.Add(5, 3)}", new { calculator }); // May have limitations with anonymous types var data = new { calc = (Func<int, int, int>)((a, b) => a + b) };

2. Required References: The engine automatically adds references to common assemblies like System.Linq, but custom assemblies might need to be explicitly included via reflection.

3. Type Compatibility: Parameters in method calls must be compatible with expected parameter types. The engine attempts to convert types when possible.

Limitations

1. Anonymous Method Limitations: Methods defined inline as lambda expressions in anonymous objects may have restricted functionality compared to methods in concrete classes.

2. Extension Method Support: Extension methods (like many LINQ methods) require the appropriate namespace to be in scope. The engine includes common namespaces by default.

3. Performance Considerations: Complex method calls and LINQ expressions require more compilation resources than simple property access.

Configuration Options

The DollarSignOptions class provides several options to customize the behavior of the expression evaluation:

var options = new DollarSignOptions { // Whether to cache compiled expressions. Defaults to true. UseCache = true, // Whether to throw exceptions on errors during evaluation. Defaults to false. ThrowOnError = false, // Custom variable resolver function. VariableResolver = variableName => /* Return value for variable */, // Custom error handler function. ErrorHandler = (expression, exception) => /* Return replacement text for errors */, // The culture to use for formatting. If null, the current culture is used. CultureInfo = new CultureInfo("en-US"), // Whether to support dollar sign syntax in templates. // When enabled, {expression} is treated as literal text and ${expression} is evaluated. // Defaults to false. SupportDollarSignSyntax = true, // Security level for expression validation (Strict, Moderate, Permissive) SecurityLevel = SecurityLevel.Moderate, // Maximum execution time for expressions in milliseconds TimeoutMs = 5000, // Cache configuration CacheSize = 1000, CacheTtl = TimeSpan.FromHours(1) };

You can also use the fluent configuration methods:

// Default options var options = DollarSignOptions.Default; // Chained configuration var secureOptions = DollarSignOptions.Default .WithStrictSecurity() .WithTimeout(TimeSpan.FromSeconds(2)) .WithCache(500, TimeSpan.FromMinutes(30)); // Pre-configured options for common scenarios var productionOptions = DollarSignOptions.Default.OptimizedForProduction(); var performanceOptions = DollarSignOptions.Default.OptimizedForPerformance(); var securityOptions = DollarSignOptions.Default.OptimizedForSecurity(); // Custom configuration var customOptions = DollarSignOptions.Create(opts => { opts.SecurityLevel = SecurityLevel.Strict; opts.TimeoutMs = 1000; opts.ThrowOnError = true; });

Error Handling

The library provides detailed exceptions for different types of errors:

try { var result = await DollarSign.EvalAsync("{nonExistentVariable + 1}", null, new DollarSignOptions { ThrowOnError = true }); } catch (VariableResolutionException ex) { Console.WriteLine($"Variable '{ex.VariableName}' not found"); Console.WriteLine($"Available variables: {string.Join(", ", ex.AvailableVariables)}"); // Provides suggestions for similar variable names } catch (ExpressionValidationException ex) { Console.WriteLine($"Security validation failed: {ex.Message}"); Console.WriteLine($"Expression: {ex.Expression}"); if (ex.Suggestion != null) Console.WriteLine($"Suggestion: {ex.Suggestion}"); } catch (ExpressionTimeoutException ex) { Console.WriteLine($"Expression timed out after {ex.Timeout}: {ex.Expression}"); } catch (CompilationException ex) { Console.WriteLine($"Compilation error: {ex.Message}"); Console.WriteLine($"Details: {ex.ErrorDetails}"); } catch (DollarSignEngineException ex) { Console.WriteLine($"General error: {ex.Message}"); }

Synchronous API

The library also provides synchronous versions of the evaluation methods:

// Using object variables var result = DollarSign.Eval("Hello, {name}!", new { name = "World" }); // Using dictionary variables var parameters = new Dictionary<string, object?> { { "value", 42 } }; var result = DollarSign.Eval("The answer is {value}.", parameters);

Performance and Security

Performance Features

• Expression Caching: Compiled expressions are cached automatically for improved performance
• Performance Metrics: Monitor cache hit rates and evaluation performance: var (totalEvaluations, cacheHits, hitRate) = DollarSign.GetMetrics(); Console.WriteLine($"Cache hit rate: {hitRate:P2}");
• Parallel Evaluation: Evaluate multiple templates concurrently: var templates = new Dictionary<string, string> { ["greeting"] = "Hello, {name}!", ["farewell"] = "Goodbye, {name}!" }; var results = await DollarSign.EvalManyAsync(templates, new { name = "World" });

Security Features

• Expression Validation: Built-in validation prevents dangerous operations
• Configurable Security Levels: Choose from Strict, Moderate, or Permissive validation
• Timeout Protection: Expressions are automatically terminated if they exceed the configured timeout
• Resource Limits: Configurable limits on expression complexity and execution time

Best Practices

• Use expression caching for repeated evaluations of the same template
• Consider security implications when allowing user-provided expressions
• Use strongly-typed objects for better performance with method calls
• Monitor performance metrics in production environments
• Clear the cache periodically if memory usage becomes a concern: DollarSign.ClearCache();

Framework Compatibility

• .NET Standard 2.1 - Compatible with .NET Framework 4.6.1+, .NET Core 2.1+
• .NET 5, 6, 7, 8, 9 - Full support for modern .NET versions
• Cross-platform - Works on Windows, Linux, and macOS