Setting the default value of a DateTime Property t

Question

Does any one know how I can specify the Default value for a DateTime property using the System.ComponentModel DefaultValue Attribute?

for example I try this:

[DefaultValue(typeof(DateTime),DateTime.Now.ToString("yyyy-MM-dd"))]
public DateTime DateCreated { get; set; }

And it expects the value to be a constant expression.

This is in the context of using with ASP.NET Dynamic Data. I do not want to scaffold the DateCreated column but simply supply the DateTime.Now if it is not present. I am using the Entity Framework as my Data Layer

Cheers,

Andrew

Answer 1

Creating a new attribute class is a good suggestion. In my case, I wanted to specify 'default(DateTime)' or 'DateTime.MinValue' so that the Newtonsoft.Json serializer would ignore DateTime members without real values.

[JsonProperty( DefaultValueHandling = DefaultValueHandling.Ignore )]
[DefaultDateTime]
public DateTime EndTime;

public class DefaultDateTimeAttribute : DefaultValueAttribute
{
    public DefaultDateTimeAttribute()
        : base( default( DateTime ) ) { }

    public DefaultDateTimeAttribute( string dateTime )
        : base( DateTime.Parse( dateTime ) ) { }
}

Without the DefaultValue attribute, the JSON serializer would output "1/1/0001 12:00:00 AM" even though the DefaultValueHandling.Ignore option was set.

Answer 2

There's no reason I can come up with that it shouldn't be possible to do through an attribute. It might be in Microsoft's backlog. Who knows.

The best solution I have found is to use the defaultValueSql parameter in the code first migration.

CreateTable(
    "dbo.SomeTable",
    c => new
        {
            TheDateField = c.DateTime(defaultValueSql: "GETDATE()")
        });

I don't like the often reference solution of setting it in the entity class constructor because if anything other than Entity Framework sticks a record in that table, the date field won't get a default value. And the idea of using a trigger to handle that case just seems wrong to me.

Answer 3

I think you can do this using StoreGeneratedPattern = Identity (set in the model designer properties window).

I wouldn't have guessed that would be how to do it, but while trying to figure it out I noticed that some of my date columns were already defaulting to CURRENT_TIMESTAMP() and some weren't. Checking the model, I see that the only difference between the two columns besides the name is that the one getting the default value has StoreGeneratedPattern set to Identity.

I wouldn't have expected that to be the way, but reading the description, it sort of makes sense:

Determines if the corresponding column in the database will be auto-generated during insert and update operations.

Also, while this does make the database column have a default value of "now", I guess it does not actually set the property to be DateTime.Now in the POCO. This hasn't been an issue for me as I have a customized .tt file that already sets all of my date columns to DateTime.Now automatically (it's actually not hard to modify the .tt file yourself, especially if you have ReSharper and get a syntax highlighting plugin. (Newer versions of VS may already syntax highlight .tt files, not sure.))

The issue for me was: how do I get the database column to have a default so that existing queries that omit that column will still work? And the above setting worked for that.

I haven't tested it yet but it's also possible that setting this will interfere with setting your own explicit value. (I only stumbled upon this in the first place because EF6 Database First wrote the model for me this way.)

Answer 4

I also wanted this and came up with this solution (I'm only using the date part - a default time makes no sense as a PropertyGrid default):

public class DefaultDateAttribute : DefaultValueAttribute {
  public DefaultDateAttribute(short yearoffset)
    : base(DateTime.Now.AddYears(yearoffset).Date) {
  }
}

This just creates a new attribute that you can add to your DateTime property. E.g. if it defaults to DateTime.Now.Date:

[DefaultDate(0)]

Answer 5

It is possible and quite simple:

for DateTime.MinValue

[System.ComponentModel.DefaultValue(typeof(DateTime), "")]

for any other value as last argument of DefaultValueAttribute specify string that represent desired DateTime value.

This value must be constant expression and is required to create object (DateTime) using TypeConverter.

Answer 6

I think the easiest solution is to set

Created DATETIME2 NOT NULL DEFAULT GETDATE()

in column declaration and in VS2010 EntityModel designer set corresponding column property StoreGeneratedPattern = Computed.