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();
Then we can run the application to test.
Summary
It's easy to create a lambda expression object based on input, output and its types.
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