Use list of structs and set value in one of these struct

Question

5.00/5 (1 vote)

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I would like to do the following:

C#

public struct MyDefinition
{
  public Int32 ID;
  public string remember;
  public Color color;
  public MyDefinition(Int32 ID, string remember, Color color)
  {
    this.ID = ID;
    this.remember = remember;
    this.color = color;
  }
}

//and here is my List of structs:
private List<MyDefinition> things = new List<MyDefinition>();

private void FillThings()
{
  things.Add(new MyDefinition(1,"",Color.Red))
  things.Add(new MyDefinition(2,"",Color.Blue))
}

public void Remember(string RememberThis)
{
  //What to do here to update things[2].remember?
}

I have tried to set the value

C#

things[2].remember = "rememberthis";

but it forgets the value.

I have also tried to implement a method in the struct

C#

public SetRemember(string remember)
{
  this.remember = remember;
}

this method indeed sets the remember value in the struct when I debug, but as soon as it leaves the method the remember string is forgotten. I read something about using Interface, but I cannot get that clear.

Any ideas?

Posted 17-Oct-13 0:41am

Mart Rijkers

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Comments

johannesnestler 17-Oct-13 10:44am

Griff's solution is very good - but in your case it seems there is no good reason to use a struct (this is .NET and not C++), so just use a class, and access/modify your objects properties as needed. For me a general rule of thumb is: There has to be a VERY good reason to create a value type, you have one?

2 solutions

Solution 1

Are you sure, that your List even has three elements? In the example you are only adding two.

Another problem could be that structs in C# are by definition value types. So it is possible that things[2].remember gets you a temporary copy of the object and sets the member in the copy. In this case you would have to do something like this:

C#

MyDefinition temp = things[2];
temp.remember = "rememberthis";
things[2] = temp;

Alternatively you could change the struct to a class. In this case the list would just store the references to the objects, so you could set the members like you originally intended.

Posted 17-Oct-13 0:57am

Freak30

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OriginalGriff · Accepted Answer · 2013-10-17T01:03:00

Firstly, there is no element at index 2, since you did not add three values. There are elements at indexes 0 and 1, but not 2...

Secondly, that is exactly what I would expect: a struct and a class have an important difference: a struct is a value type, a class is a reference type.
You can't write:

C#

things[index].remember = "rememberthis";

because the system will complain:

"Cannot modify the return value of 'System.Collections.Generic.List<complexfilterdemo.frmtester.mydefinition>.this[int]' because it is not a variable"</complexfilterdemo.frmtester.mydefinition>

And that makes sense, because it is a copy of a value type! If you want to "remember" your change, then you need to do this:

C#

MyDefinition md = things[1];
md.remember = "hello";
things[1] = md;

Because when you get or set the value of a value type you are copying the type instance, not getting another reference to the same instance. For proof, try this:

C#

Console.WriteLine("'{0}'", things[1].remember);
MyDefinition md = things[1];
md.remember = "hello";
Console.WriteLine("'{0}':'{1}'", things[1].remember, md.remember);
things[1] = md;
Console.WriteLine("'{0}':'{1}'", things[1].remember, md.remember);
md.remember = "goodbye";
Console.WriteLine("'{0}':'{1}'", things[1].remember, md.remember);

You will get:

''
'':'hello'
'hello':'hello'
'hello':'goodbye'

Which only makes sense if the values are being copied, not referenced.