FastExpressionCompiler: Faster Expression Tree Compilation

Why CompileFast Targets a Real Bottleneck

FastExpressionCompiler addresses a specific C# infrastructure cost: `Expression.Compile()` is slow when an application creates delegates from `Expression<TDelegate>` trees. The README says expression compilation appears in IoC and dependency injection containers, serializers, ORMs, and object-oriented mappers. That places the project below application code, inside libraries that may build the same kind of expression many times during startup, configuration, query construction, or request processing.

The normal compiler path creates a `DynamicMethod` associated with an anonymous assembly. The project documentation describes that sandbox arrangement as safe for partially trusted code, but adds runtime overhead. FastExpressionCompiler offers `.CompileFast()` as an alternative extension method. Its stated goal is to reduce the time needed to create a delegate, with additional attention to allocation during compilation.

This is not a general C# compiler or a replacement for writing ordinary delegates by hand. It is aimed at code that already depends on expression trees. If an application has one static predicate and never rebuilds it, the compilation cost is unlikely to justify adding this dependency. Repeated construction or library-level reuse is the stronger use case.

CompileFast and LightExpression Packages

The repository publishes two main package families. `FastExpressionCompiler` is the Expression Trees implementation, with source packages that expose the project sources and internal variants that make public code internal. `FastExpressionCompiler.LightExpression` provides the lightweight LightExpression alternative and follows the same package split. The README names six relevant NuGet packages: `FastExpressionCompiler`, `FastExpressionCompiler.src`, `FastExpressionCompiler.Internal.src`, `FastExpressionCompiler.LightExpression`, `FastExpressionCompiler.LightExpression.src`, and `FastExpressionCompiler.LightExpression.Internal.src`.

That distinction matters during adoption. The standard package is appropriate when the application uses `System.Linq.Expressions` expression trees. The LightExpression package is relevant only when the application is already using that alternative representation. The README does not provide a detailed migration guide or a complete comparison of the two APIs, so package selection should follow the expression model already present in the consuming library.

The source packages may be useful when internal access or source inspection is part of the integration decision. The repository also includes a solution, source directories under `src/`, tests under `test/`, and build scripts, but the README excerpt does not document a required source build for normal consumers.

Install and Compile a First Expression

The README does not document a specific installation command, so there is no command to copy without guessing. It directs consumers to the NuGet packages listed above. For an application using ordinary C# Expression Trees, the relevant package is `FastExpressionCompiler`.

A minimal first use is an expression with captured constructor arguments. The README benchmark defines the expression in this form:

var a = new A();
var b = new B();
Expression<Func<X>> e = () => new X(a, b);

An application would call the project’s extension method on that expression, then invoke the returned delegate. The README identifies the extension as `.CompileFast()`. Follow the package’s API documentation or source package rather than inventing an `using` directive.

The expression’s result is a delegate for `X`. Captured variables such as `a` and `b` are part of the closure state used when the delegate runs. This makes the example useful for checking closure handling as well as compilation speed.

What the README Benchmarks Actually Show

The benchmark section reports results on Windows 11 with an Intel Core i9-8950HK, .NET SDK 9.0.203, and .NET 9.0.4. It says the benchmarks were updated to .NET 9.0. That context is necessary because the README explicitly warns that actual performance varies with platform, expression complexity, closures, and nested lambdas.

For the hoisted constructor expression with two closure arguments, the compiling table lists `CompileFast` at 3.183 microseconds and `Compile` at 147.312 microseconds. The allocation column lists 1.23 KB for `CompileFast` and 4.48 KB for `Compile`. These are reported measurements for one expression, one machine, and one runtime configuration. They do not establish the same ratio for every consumer.

The invocation table shows a different concern. `FastCompiledLambda` is reported at 7.962 nanoseconds, while `CompiledLambda` is 7.853 nanoseconds and the direct constructor call is 6.055 nanoseconds. In other words, faster compilation did not make every invocation faster in this example. The project’s headline claim is about compilation, and the README qualifies execution performance separately. Teams should evaluate both stages when expressions are built frequently and invoked frequently.

Expression.Compile Versus CompileFast

The direct alternative is the built-in `.Compile()` method. Both APIs accept an expression tree and produce a delegate, so the difference is primarily the compilation mechanism used by the library. The README explains that `Expression.Compile()` uses a `DynamicMethod` associated with an anonymous assembly, which supports sandboxed execution but incurs overhead. `CompileFast()` is the project’s alternative compilation route.

A manually created delegate is another practical alternative when the operation is fixed. If a library author knows the exact method and arguments in advance, hand-written delegate construction can remove expression compilation from the path entirely. The cost is loss of the flexibility supplied by an expression tree. FastExpressionCompiler is more relevant when expression structure is assembled dynamically but the resulting delegate still needs to be invoked.

The LightExpression packages provide a different internal choice for applications that already use LightExpression. The README describes LightExpression as a lightweight alternative, but it does not explain enough of its expression syntax or compiler boundaries to recommend it independently. The actual decision should follow the expression type already used by the consuming code.

Limits, Compatibility, and Upgrade Cost

The main limitation is that the project does not remove the need to validate runtime support for a particular expression. The README says results depend on platform, expression complexity, closures, nested lambdas, and other factors. It does not claim that every expression has the same speedup, nor does it claim that generated delegates always execute faster. The invocation benchmark includes a case where the ordinary compiled lambda is marginally faster than the fast-compiled lambda.

The documented targets are .NET 8, .NET 10, .NET Standard 2.1, and .NET Framework 4.7.2. Consumers must check that the selected package and release work with their application’s target framework, dependency graph, and runtime. The project uses C#, and its central integration point is an extension method on expression trees.

Maintenance status is favorable by the repository facts: the repository is not archived, and the last push was on 2026-10-07. The listed releases include v6.0.0 on 2026-10-07, v5.4.1 on 2026-04-13, and v5.4.0 on 2026-03-30. That supports recent release activity, but it does not establish compatibility for a particular consuming application. The project is MIT licensed. The license text should be reviewed for distribution and notice obligations, without treating this article as legal advice. An upgrade can also require checking changes in expression handling across major versions such as v6.0.0.

Editorial conclusion

FastExpressionCompiler is a reasonable choice for infrastructure libraries that compile many expression trees and can accept its MIT-licensed dependency and platform coverage. It is less suitable for applications that compile an expression once or need behavior not documented by the project. The first checks should be package compatibility with the target runtime, support for the exact expression constructs in the application, and measured results for the project’s own expression shapes.

Frequently asked questions

Official sources

1. dadhi/FastExpressionCompiler on GitHub
2. Issues
3. License: MIT
4. README
5. Releases