How to get the default value of an object property?

Question

Some code:

foreach (System.Reflection.PropertyInfo pi in myObject.GetType().GetProperties())
{
    if (pi.CanWrite)
    {
        object value = pi.GetValue(Properties, null);

        // if (value is not default)
        // {
        X.addAttribute(pi.Name, value);
        // }
    }
}

What I'm trying to do is not-call the line 'X.addAttribute...' if the property is at its DefaultValue. I assume there's some way of getting the DefaultValue of a property so I can do a comparison?

For my purpose I am trying to get the 'default' value as defined by DefaultValueAttribute.

Any help is appreciated, cheers.

Answers will depend based on how you define "default value". The two obvious definitions are CLR-Default (`null` for reference types, 0 for value types) or the `DefaultValueAttribute`. Can you edit your question to clarify which you are interested in? — Paul Turner, Feb 16 '10 at 12:01
Hi, apologies; I should have specified. It was default as defined via DefaultValueAttribute. — Wex, Feb 16 '10 at 12:05

score 5 · Answer 1 · answered Feb 16 '10 at 11:41

Below is the method I use to get a default value of any runtime-type it will return 'null' for non value types otherwise it will return the default value type (it includes caching of value types for extra perf):

private static readonly Dictionary<Type, object> DefaultValueTypes 
    = new Dictionary<Type, object>();

public static object GetDefaultValue(Type type)
{
    if (!type.IsValueType) return null;

    object defaultValue;
    lock (DefaultValueTypes)
    {
        if (!DefaultValueTypes.TryGetValue(type, out defaultValue))
        {
            defaultValue = Activator.CreateInstance(type);
            DefaultValueTypes[type] = defaultValue;
        }
    } 

    return defaultValue;
}

Thanks, but my system uses some non-value types. This is a handy bit of code to have anyway :). — Wex, Feb 16 '10 at 11:58
Alternatively, if the type is known at compile-time, the `default` keyword can be used. — Paul Turner, Feb 16 '10 at 11:58

score 3 · Accepted Answer · answered Feb 16 '10 at 11:35

3

Assuming you're trying to get the DefaultValue attribute, use GetCustomAttributes on your PropertyInfo objects.

answered Feb 16 '10 at 11:35

Sylvestre Equy

385
1
8

Cheers. This is the bit of info I needed for my purpose. – Wex Feb 16 '10 at 11:57

Asad · Answer 3 · 2010-02-16T11:38:30.780

-3

First, You must always initialize the field with value you want as a default.

Something similar

    class UnitManager
    {
      private int value;

      [DefaultValue("0")]
      public int Value
      {
       get { return this.value; }
       set { this.value = value; } 
      }
    }

Then simply use

    UnitManager manager = new UnitManager()

    int startValue = manager.Value;

edited Feb 16 '10 at 11:38

answered Feb 16 '10 at 11:31

Asad

21,468
17
69
94

How to get the default value of an object property?

3 Answers3

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