DefaultValue Attribute in C#

Question

I want to use on DefaultValue Attribute to define default value for custom class that I write in my App. the class gives in his constractor a string. I write the follow:

[DefaultValue(Type.GetType("MyClass"),"hello world")]

but when I try to run this App. I give error:

"An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type".

can anyone explain me what the problem?

Peter Taylor · Accepted Answer · 2011-01-17 13:30:15Z

6

You're using Type.GetType("MyClass") where you should have typeof(MyClass).

answered Jan 17, 2011 at 13:30

Peter Taylor

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4 Comments

RRR Over a year ago

it works, but now I can't success to give the value of this attribute. what sould I write to give it?

Peter Taylor Over a year ago

I think you need to explain in detail what you're trying to do.

RRR Over a year ago

I try to build an enum that the his values are not int or char. so I write before each value - [DefaultValue(Type.GetType("MyClass"),"11:00:00")] and in the ctor of the MyClass I parss the string. but now how can I recive for each value the value that I write in the DefaultValue Attribute?

Peter Taylor Over a year ago

In C# the enum values must be an integer type. stackoverflow.com/questions/469287/… might be helpful.

WraithNath · Accepted Answer · 2011-01-17 13:29:54Z

2

I suspect its the Type.GetType("MyClass");

can you try typeof(MyClass) instead, passing the type and not a string?

answered Jan 17, 2011 at 13:29

WraithNath

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2 Comments

RRR Over a year ago

it works, but now I can't success to give the value of this attribute. what sould I write to give it?

WraithNath Over a year ago

Are you getting an error? i just read this: eggheadcafe.com/software/aspnet/32319410/… it explains a little on when the value would be visible. I must admit I have not used DefaultValueAttributes before put I might be able to help if you can give a little more detail. The other guys on this thread may be able to help also if they see this comment

brunnerh · Accepted Answer · 2011-01-17 15:00:48Z

1

Type.GetType() is a method (i.e. not a constant expression), as the others said, use typeof.

[DefaultValue(typeof(MyClass),"Convertible String")]

Edit: To enable the conversion of the string to your custom class you need to associate a TypeConverter with it, see the examples-section of this documentation to get an idea of how to do so.

edited Jan 17, 2011 at 15:00

answered Jan 17, 2011 at 13:31

brunnerh

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2 Comments

RRR Over a year ago

it works, but now I can't success to give the value of this attribute. what sould I write to give it?

brunnerh Over a year ago

I haven't used the DefaultValue attribute much, but when i did, i never needed to give it a type, i just entered some value like this for a bool: [DefaultValue(false)], since you want to use a more complex type you should look into how it tries to convert the value, maybe your class needs to implement an interface for it to work.

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