ExpressionTools

ExpressionTools is a library that helps you to handle C# lambda expression. It is written in C# and thanks to .Net Standard, it is cross platform.

The project is based on System.Linq.Expressions and uses the C# Roslyn compiler.

Don't hesitate to post issues, pull requests on the project or to fork and improve the project.

Project dashboard

Package	Nuget.org	Pre-release
SoloX.ExpressionTools.Parser
SoloX.ExpressionTools.Transform

License and credits

ExpressionTools project is written by Xavier Solau. It's licensed under the MIT license.

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Installation

You can checkout this Github repository or you can use the NuGet package:

Install using the command line from the Package Manager:

Install-Package SoloX.ExpressionTools.Parser -version 1.0.4
Install-Package SoloX.ExpressionTools.Transform -version 1.0.4

Install using the .Net CLI:

dotnet add package SoloX.ExpressionTools.Parser --version 1.0.4
dotnet add package SoloX.ExpressionTools.Transform --version 1.0.4

Install editing your project file (csproj):

<PackageReference Include="SoloX.ExpressionTools.Parser" Version="1.0.4" />
<PackageReference Include="SoloX.ExpressionTools.Transform" Version="1.0.4" />

How to use it

Note that you can find code examples in this repository in this location: src/examples.

Parse a C# Lambda expression

Simple

A few lines of code are actually needed to parse a textual C# lambda expression. Let's say that we want to parse a simple lambda expression like "(int x) => x + 1" with x as an integer:

// Set the expression to parse
var expToParse = "(int x) => x + 1";

// We need to create the parser.
var expressionParser = new ExpressionParser();

// We can just parse the expression.
var lambdaExpression = expressionParser.Parse(expToParse);

// Or we can parse the expression specifying the type of the lambda expression.
var expression = expressionParser.Parse<Func<int, int>>(expToParse);

The Parse method will return an expression tree (from System.Linq.Expressions) reflecting the given textual C# lambda expression.

More precisely, The resulting expression object is based on System.Linq.Expressions.LambdaExpression. It means that you can then use the Compile method to compute a Delegate in order to call the lambda at full speed.

Using a IParameterTypeResolver

Unfortunately, things can get a little bit more complicated if you want to parse the same expression without specifying the parameter type giving us a expression like "x => x + 1". In this case you will need to provide to the ExpressionParser a IParameterTypeResolver that will resolve the type of the parameter x as an integer:

// Set the expression to parse
var expToParse = "x => x + 1";

// We need to create the parser with a DictionaryParameterTypeResolver that will resolve the
// parameter name using the given Dictionary.
var expressionParser = new ExpressionParser(
    parameterTypeResolver: new DictionaryParameterTypeResolver(new Dictionary<string, Type>()
    {
        { "x", typeof(int) },
    }));

// We can just parse the expression.
var expression = expressionParser.Parse<Func<int, int>>(expToParse);

Using a ITypeNameResolver

Let's now use a static method Max that is defined in System.Math with a full qualified name like "(double x, double y) => Math.Max(x, y)". To support this use case, you will need to provide a ITypeNameResolver that will resolve the Math as System.Math class:

// Set the expression to parse
var expToParse = "(double x, double y) => Math.Max(x, y)";

// We need to create the parser with a TypeNameResolver that will resolve type name with
// the given System.Math class.
var expressionParser = new ExpressionParser(
    typeNameResolver: new TypeNameResolver(typeof(Math)));

// We can just parse the expression.
var expression = expressionParser.Parse<Func<double, double, double>>(expToParse);

Using a IMethodResolver

Now we want our expression to use a static method Max that is defined in System.Math omitting the class name prefix like "(double x, double y) => Max(x, y)". To support this use case, you will need to provide a IMethodResolver that will resolve the Max as System.Math.Max:

// Set the expression to parse
var expToParse = "(double x, double y) => Max(x, y)";

// We need to create the parser with a StaticMethodResolver that will resolve methods with
// the System.Math class.
var expressionParser = new ExpressionParser(
    methodResolver: new StaticMethodResolver(typeof(Math)));

// We can just parse the expression.
var expression = expressionParser.Parse<Func<double, double, double>>(expToParse);

Expression transformation

Inline C# Lambda expression

One parameter expression

In the case where we would like the replace the parameter a from one expression like "a => a * 2" by another expression like "b => b + 1" resulting in "b => (b + 1) * 2", we can use the SingleParameterInliner.

// Setup the expressions to use as input
Expression<Func<int, int>> expressionToAmend = a => a * 2;
Expression<Func<int, int>> expressionToInline = b => b + 1;

// create the expression parameter in-liner.
var inliner = new SingleParameterInliner();

// Amend the given expression replacing parameter 'a' with the expression to in-line resulting in the
// lambda 'b => (b + 1) * 2'.
var inlinedExpression = inliner.Amend<Func<int, int>, Func<int, int>>(expressionToInline, expressionToAmend);

Multi parameter expression

In the case where we would like to amend an expression like "(a, b) => a * b" replacing the two parameter 'a' and 'b' with two another expressions:

• using "c => c + 1" to replace 'a'
• and using "d => d - 1" to replace 'b'

all this resulting in "(c, d) => (c + 1) * (d - 1)", we can use the MultiParameterInliner.

All it needs is a ParameterResolver that will be used to get the expression to in-line instead of the parameter itself:

// Setup the expressions to use as input
Expression<Func<int, int>> expressionToAmend = (a, b) => a * b;
Expression<Func<int, int>> expressionToInlineForA = c => c + 1;
Expression<Func<int, int>> expressionToInlineForB = d => d - 1;

// create the expression in-liner.
var inliner = new ExpressionInliner();

// Setup the resolver telling that 'a' must be replaced by in-lined 'c => c + 1' lambda
// and that 'b' must be replaced by in-lined 'd => d - 1' lambda.
var resolver = new ParameterResolver()
    .Register("a", expressionToInlineForA)
    .Register("b", expressionToInlineForB);

// Amend the given expression replacing parameters resulting in the lambda '(c, d) => (c + 1) * (d - 1)'.
var inlinedExpression = inliner.Amend<Func<int, int>, Func<int, int>>(resolver, expressionToAmend);

Inline Constant expression

Let's say that we need to convert a Lambda Expression using an external variable like this:

// Here is a variable we are going to use in a Lambda Expression.
var externalValue = 0.01d;

// The expression is multiplying the input with the variable 'externalValue'.
Expression<Func<double, double>> expToInline = i => i * externalValue;

The use case here is that we want to convert the expression in-lining the actual value of the variable like i => i * 0.01d.

It has never been simpler with the ConstantInliner: all we need is to call its Amend method:

// Create the constant in-liner.
var inliner = new ConstantInliner();

// Amend the expToInline.
var exp = inliner.Amend(expToInline);

// That's yet, 'exp' is equal to 'i => i * 0.01d'

Resolve Property Name

In order to get the name of a property from a lambda like i => i.MyProperty we can use the PropertyNameResolver:

// Create a instance of the resolver.
var resolver = new PropertyNameResolver();

// Resolve the property name of the given lambda.
var name = resolver.GetPropertyName<IMyType, string>(x => x.MyProperty);

// Here name is set to "MyProperty".

Expression serializer

To serialize your expression as a string, you can use the Serialize method like this:

using SoloX.ExpressionTools.Transform;

Expression<Func<double, double, double>> expression = (a, b) => Math.Pow(a, b);

var txt = expression.Serialize();

// will give you the string:
// "(a, b) => Math.Pow(a, b)"