How do you give a C# Auto-Property a default value?

Question

How do you give a C# Auto-Property a default value? I either use the constructor, or revert to the old syntax.

Using the Constructor:

class Person 
{
    public Person()
    {
        Name = "Default Name";
    }
    public string Name { get; set; }
}

Using normal property syntax (with a default value)

private string name = "Default Name";
public string Name 
{
    get 
    {
        return name;
    }
    set
    {
        name = value;
    }
}

Is there a better way?

Answer 1

Have you tried using the DefaultValueAttribute or ShouldSerialize and Reset methods in conjunction with the constructor? I feel like one of these two methods is necessary if you're making a class that might show up on the designer surface or in a property grid.

Answer 2

To give auto implemented properties a default value, you'd have to do it in the constructor.

Answer 3

DefaultValueAttribute ONLY work in the vs designer. It will not initialize the property to that value.

Answer 4

Sometimes I use this, if I don't want it to be actually set and persisted in my db:

class Person
{
    private string _name; 
    public string Name 
    { 
        get 
        {
            return string.IsNullOrEmpty(_name) ? "Default Name" : _name;
        } 

        set { _name = value; } 
    }
}

Obviously if it's not a string then I might make the object nullable ( double?, int? ) and check if it's null, return a default, or return the value it's set to.

Then I can make a check in my repository to see if it's my default and not persist, or make a backdoor check in to see the true status of the backing value, before saving.

Hope that helps!

Answer 5

Personally, I don't see the point of making it a property at all if you're not going to do anything at all beyond the auto-property. Just leave it as a field. The encapsulation benefit for these item are just red herrings, because there's nothing behind them to encapsulate. If you ever need to change the underlying implementation you're still free to refactor them as properties without breaking any dependent code.

Hmm... maybe this will be the subject of it's own question later

Answer 6

@Joel: data binding and other reflection-based tools often expect properties rather than fields.

Answer 7

When you inline an initial value for a variable it will be done implicitly in the constructor anyway.

I would argue that this syntax is best practice:

class Person 
{
    public Person()
    {
        //do anything before variable assignment

        //assign initial values
        Name = "Default Name";

        //do anything after variable assignment
    }
    public string Name { get; set; }
}

As this gives you more clear control of the order values are assigned.

Answer 8

You should not add a default to the constructor; this will mean that in the creation of the object the property will have to be assigned to twice (once as null, then again in the constructor). If you require a default value for a property it should broken out as a normal property (or possibly deferred to a builder in a creational pattern).

One other option is to do what ASP.Net does and define defaults via an attribute:

http://msdn.microsoft.com/en-us/library/system.componentmodel.defaultvalueattribute.aspx

But again, I would simply break out the property as this is the clearest and most efficient option.

Answer 9

class Person 
{ 
    private string _name;  

    [DefaultValue(true)]  
    public string Name  
    {  
        get  { return _name; }     
        set { _name = value; }  
    } 
}

Answer 10

little complete sample:

private bool bShowGroup ;
[Description("Show the group table"), Category("Sea"),DefaultValue(true)]
public bool ShowGroup
{
    get { return bShowGroup; }
    set { bShowGroup = value; }
}

Answer 11

[DefaultValue(true)]
dit not fill the property