Create Action to assign property from name of a property provided by string

Question

What I'm trying to achieve is to create Action to assign a value to the object property defined by a string. What i have come up with so far is:

void Main()
{
    var startPropertyName= "StartTime";
    var endPropertyName= "EndTime";


    var myAction = AssignValueToProperty<MyClass>(startPropertyName, DateTime.Today);

    var myObject = new MyClass();
    myAction(myObject);
    myObject.StartTime.Dump();

}

public static Action<T> AssignValueToProperty<T>(string propertyName, DateTime value)
{
    var arg = Expression.Parameter(typeof(T));
    var property = Expression.Property(arg, propertyName);

    var cons = Expression.Constant(value, typeof(DateTime));

    var body = Expression.Assign(property, cons);
    var exp = Expression.Lambda<Action<T>>(body, new ParameterExpression[] { arg });
    return exp.Compile();
}

public class MyClass
{
    public DateTime StartTime { get; set; }
    public DateTime EndTime { get; set; }
}

But i would like to pass the DateTime parameter during calling the Action not during creating it. And possibly add another parameter for EndTime Property.

Which part of your problem are you struggling with? You just want to add another parameter to the lambda, so why not do that? — canton7, Jan 13 '20 at 10:57
I want to create an Action like this `AssignValueToProperty(startPropertyName)` and call it like this: `myAction(myObject,DateTime.Now);` — mdziadowiec, Jan 13 '20 at 10:59

canton7 · Accepted Answer · 2020-01-13T11:20:34.560

2

You were most of the way there. Just declare another ParameterExpression for your new parameter.

public static Action<T, DateTime> AssignValueToProperty<T>(string propertyName)
{
    var arg = Expression.Parameter(typeof(T), "arg");
    var startTime = Expression.Parameter(typeof(DateTime), "startTime");
    var property = Expression.Property(arg, propertyName);

    var body = Expression.Assign(property, startTime);
    var exp = Expression.Lambda<Action<T, DateTime>>(body, new ParameterExpression[] { arg, startTime });
    return exp.Compile();
}

edited Jan 13 '20 at 11:20

answered Jan 13 '20 at 11:03

canton7

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It works, you just missed quotation marks in `var startTime = Expression.Parameter(typeof(DateTime), "startTime");` and actualy asign the starttime i the Assign method `var body = Expression.Assign(property, startTime);` – mdziadowiec Jan 13 '20 at 11:09
Oops, knocked that out on the way to a meeting. Fixed! – canton7 Jan 13 '20 at 11:21
I think the names of the paramters aren't even necessary right? – mdziadowiec Jan 13 '20 at 11:24
1

They're not, but it helps a lot when debugging. If you look at the `DebugView` property of an expression in the debugger, it uses the names in the generated pseudo-code it shows you. – canton7 Jan 13 '20 at 11:30
is there a way to infer type of T dynamically so i could call `var myAction = AssignValueToProperty(startPropertyName, DateTime.Today);` instead of `var myAction = AssignValueToProperty(startPropertyName, DateTime.Today);` because i know the type only during runtime. I was looking at this example [link](https://stackoverflow.com/a/25812889/2458980) but the "Using Expresion Tree" part actually Converts the parameter to the type provided by the user input – mdziadowiec Jan 13 '20 at 14:10
At the point that you compile the expression, you need to know the type of the object you're setting the property on. You don't necessarily need to know it as a `T` (it could be a `Type` object), but it does need to be known – canton7 Jan 13 '20 at 14:16

Create Action to assign property from name of a property provided by string

1 Answers1