Afk.Expression 2.1.0

Expression

Afk.Expression is an expression parser/evaluation. Expression is parse only once and use to evaluate differents inputs (variables, constants, functions). This library can be used to generate SQL Expression to be used with Entity Framework.

Version

The library is compatible with netstandard2.0

NUGET

The easiest way to install is by using NuGet.

HowTo

This library allows you to enter complex expressions which will be evaluated and calculated on demand.

ExpressionEval eval = new ExpressionEval("5 * (3 + 8)");
Console.WriteLine(eval.Evaluate());
>>> 55

String and boolean expression can be evaluated too

ExpressionEval eval = new ExpressionEval("'one ' + 'and' + ' two'");
Console.WriteLine(eval.Evaluate());
>>> one and two

ExpressionEval eval = new ExpressionEval("true or false");
Console.WriteLine(eval.Evaluate());
>>> true

Operators

By default the ^ is the logical exclusive OR operator. You can change its behavior by using the operator type Arithmetic

ExpressionEval expr1 = new ExpressionEval("4^2", OperatorType.Arithmetic);
Console.WriteLine(eval.Evaluate());
>>> 16

Variables

Expression allows you to use variables evaluated on execution

ExpressionEval eval = new ExpressionEval("5 * x2 + y");
eval.AddVariable("x"); eval.AddVariable("y");
eval.UserExpressionEventHandler += (s, e) => {
    if (e.Name == "x")
        e.Result = 8d;
    else if (e.Name == "y")
        e.Result = 5d;
}; 
Console.WriteLine(eval.Evaluate());
>>> 325

Constants

Constants can be added to the expression

ExpressionEval eval = new ExpressionEval("pi");
eval.AddConstant("pi", 3.14);
Console.WriteLine(eval.Evaluate());
>>> 3.14

Functions

External function can be defined

    private void OnFunctionHandler(object sender, UserFunctionEventArgs e)
    {
        if (e.Name == "Concat")
        {
            e.Result = string.Join("", e.Parameters);
        }
    }

ExpressionEval eval = new ExpressionEval("Concat('The ', 'dogs ', 'barks') + '.'");
eval.AddFunctions("Concat");

eval.UserFunctionEventHandler += OnFunctionHandler;
Console.WriteLine(eval.Evaluate());
>>> The dogs barks

EF Core

Expression can be used to generate System.Linq.Expressions.Expression to request DbContext. You need to define a ILambdaExpressionProvider which provides expression on your entity.

class MyLambdaProvider : ILambdaExpressionProvider
{
    public Expression GetExpression(ParameterExpression parameter, string propertyName)
    {
        if (propertyName.Equals("Name", StringComparison.InvariantCultureIgnoreCase))
        {
            return System.Linq.Expressions.Expression.Property(parameter, nameof(Student.Name));
        }
        else if (propertyName.Equals("Age", StringComparison.InvariantCultureIgnoreCase))
        {
            ...
        }
        return null;
    }

Gets the expression with the extension method ToLambda on your expression :

ExpressionEval eval = new ExpressionEval("name='john'", CaseSensitivity.None);
eval.UserExpressionEventHandler += evalProperties.Evaluate;

var lambda = eval.ToLambda<Student, bool>(new MyLambdaProvider());

var result = myContext.Students.Where(lambda).ToList();

Overide operators

The ILambdaExpressionProvider enables you to override operators.

By default the like operator use the Contains method translate to CHARINDEX sql function. To overide this operator use the following definition :

class MyLambdaProvider : ILambdaExpressionProvider
{
    public Expression GetExpression(Expression left, string operand, Expression right)
    {
        if (operand == "like")
        {
            var method = typeof(DbFunctionsExtensions).GetMethod("Like", new[] { typeof(DbFunctions), typeof(string), typeof(string) });
            MethodCallExpression like = System.Linq.Expressions.Expression.Call(method, Expression.Constant(EF.Functions), left, right);
            return like;
        }
        return null;
    }
}

ExpressionEval eval = new ExpressionEval("name like '8000%'", CaseSensitivity.None);
eval.UserExpressionEventHandler += evalProperties.Evaluate;

var lambda = eval.ToLambda<Student, bool>(new MyLambdaProvider());

Console.WriteLine(lambda.ToString());
>>> "s => value(Microsoft.EntityFrameworkCore.DbFunctions).Like(s.Name, \"8000%\")"

External Links