c# set variable value based on string

Question

I have a class with a list of variables (shortened to one "a" here) - I'm trying to create a way to populate a variable based on two string inputs, so I can just use, for instance:

SET_VAL("a", "value_for_a")

Which I'd like to get the MessageBox showing just "value_for_a"

but I'm tripping up trying to use reflection, I'm sure this is possible, I know it's possible to get a value from a variable using a string to find it's name, cos every time I search for this that's all I seem to get, but why doesn't the following work? The method SetValue is asking for objects as parameters when I just want to pass two strings?

public class MySystem_SubsNote
   {
       public string a;

       public void SET_VAL(string mytype, string myvalue)
       {
            this.GetType().GetField(mytype).SetValue(myvalue);
            MessageBox.Show(a);
       }
   }

Why not just have a Dictionary<string,string> rather than any specific variables? — Damien_The_Unbeliever
– Damien_The_Unbeliever, Commented May 4, 2018 at 7:43
Possible duplicate of Setting a property by reflection with a string value — Peter B
– Peter B, Commented May 4, 2018 at 7:44
FieldInfo.SetValue requires 2 parameters, the instance on which to set the field and the value, you're only passing the value. Try .SetValue(this, myvalue); — Lasse V. Karlsen
– Lasse V. Karlsen, Commented May 4, 2018 at 7:45
When saying "it doesn't work", tell us why you think so, if you get an error message, post the error message. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented May 4, 2018 at 7:47

MakePeaceGreatAgain · Accepted Answer · 2018-05-04 07:52:36Z

4

As mentioned from Lasse in the comments SetValue expects two parameters, the first being the instance of which to set the value, the second the new value. So this should do it:

public void SET_VAL(string mytype, string myvalue)
{
    this.GetType().GetField(mytype).SetValue(this, myvalue);
    MessageBox.Show(a);
}

answered May 4, 2018 at 7:52

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4 Comments

jamheadart Over a year ago

Thanks, this makes sense but now I'm getting Object reference not set to an instance of an object. on this line. I don't understand since I've set an instance of it in another method, and am using this - when highlighted - this shows the object/class type

MakePeaceGreatAgain Over a year ago

then I suppose GetField does return null. Maybe you have a typo in your field-name?

jamheadart Over a year ago

I just recreated this as a mini-project and it worked fine so I think there may be a hidden space in my field-name on my large project (it's parsed separately from a huge string list) - thanks for the advice, marking this as best answer since it definitely works!

jamheadart Over a year ago

Just FYI it wasn't the field that was an issue, I had replaced "GetField" with "GetProperty" while I was trying different things and forgot to change it back :/

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