Posted on Mar 29, 2023

C#: Pass Lambda Expression to Select LINQ at Runtime

In the previous article, I explain how to pass the lambda string to Where clause. In this article, I do pass it to the Select extension method.

Create Classes

I used List<string> before, but let's create the class to test several different scenario.

 public class MyClass public class MyClass { public int Id { get; set; } public string Name { get; set; } = string.Empty; public int Age { get; set; } public string Country { get; set; } = string.Empty; }

Select LINQ for string

The Select method takes Func<TSource, TResult> delegate. I create List<MyClass> as an input, so the TSource is MyClass this time.

Then how about TResult? It's depends if I want to select a string property of an int property.

 ParameterExpression param = Expression.Parameter(typeof(MyClass), "x"); Expression expression = new ExpressionParser( new ParameterExpression[] { param }, "x.name", null, new ParsingConfig()) .Parse(typeof(string)); Expression<Func<MyClass, string>> lambda = Expression.Lambda<Func<MyClass, string>>(expression, param);

The input parameter is MyClass and I name it as x. Then the query is x.name to return name. Of course, the return type is string type.

Let's create a list and try the code.

 List<MyClass> inputs = new() { new () { Id = 1, Name = "1" }, new () { Id = 2, Name = "2" } }; List<string> result = inputs.AsQueryable().Select(lambda).ToList();

The results is:

Select LINQ for object

If we don't know which property to select yet, we can specify object.

 string query = Console.ReadLine(); ParameterExpression param = Expression.Parameter(typeof(MyClass), "x"); Expression expression = new ExpressionParser( new ParameterExpression[] { param }, query, null, new ParsingConfig()) .Parse(typeof(object)); Expression<Func<MyClass, object>> lambda = Expression.Lambda<Func<MyClass, object>>(expression, param); //List<string> inputs = new() { "1", "2" }; //var result = inputs.AsQueryable().Where(lambda).ToList(); List<MyClass> inputs = new() { new () { Id = 1, Name = "1" }, new () { Id = 2, Name = "2" } }; List<object> result = inputs.AsQueryable().Select(lambda).ToList();