Default Value attribute with a dictionary

Question

I am trying to use the defaultvalue attribute Default Value Documentation

So I know how to do a default attribute with a string

 [DefaultValue("This is the message")]

But how do I do a default value for a dictionary ?

I tried this

   [DefaultValue(new Dictionary<string, string>() { { "Test", "test" } })]

but I get this error

Error CS0182 An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

You can't. Attribute parameters have to be const expressions, and constructing an object isn't. There is no way to accomplish what you want if you have to use `DefaultValueAttribute`. — Lasse V. Karlsen, Jul 13 '20 at 13:38
I am surprised [there is no good](https://www.google.com/search?q=Error+CS0182+An+attribute+argument+must+be+a+constant+expression,+typeof+expression+or+array+creation+expression+of+an+attribute+parameter+type+site:stackoverflow.com) canonical answer for this error. For what you need `DefaultValueAttribute` with dictionary as a value? Serialization? Certain control (winforms? wpf?)? Something else? — Sinatr, Jul 13 '20 at 14:10

score 2 · Accepted Answer · answered Jul 13 '20 at 13:55

2

You can't use Dictionary in any attribute, but you can try to add your own attribute with key + value.

I don't know how do you want to use it, but I'd recommend trying something like:

public class DictionaryDefaultAttribute : DefaultValueAttribute
{
    public DictionaryDefaultAttribute(string key, string value) 
        : base(new Dictionary<string, string>() {{key, value}})
    {
    }
}

answered Jul 13 '20 at 13:55

Aleksei Brovko

36
2

If you have a Dynamic Dictionary, the following code also worked for me. ``` [DefaultValue("{\"Field1\": \"Value1\",\"Field2\": \"Value2\",\"Field3\":\"Value3\"}")] [Required] public Dictionary Fields { get; set; }``` – tRuEsAtM Nov 14 '22 at 19:09

Default Value attribute with a dictionary

1 Answers1