Entity Framework 6 Code first Default value

Question

You can do it by manually edit code first migration:

public override void Up()
{    
   AddColumn("dbo.Events", "Active", c => c.Boolean(nullable: false, defaultValue: true));
} 

It's been a while, but leaving a note for others. I achieved what is needed with an attribute and I decorated my model class fields with that attribute as I want.

[SqlDefaultValue(DefaultValue = "getutcdate()")]
public DateTime CreatedDateUtc { get; set; }

Got the help of these 2 articles:

• EF on CodePlex
• Andy Mehalick blog

What I did:

Define Attribute

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class SqlDefaultValueAttribute : Attribute
{
    public string DefaultValue { get; set; }
}

In the "OnModelCreating" of the context

modelBuilder.Conventions.Add( new AttributeToColumnAnnotationConvention<SqlDefaultValueAttribute, string>("SqlDefaultValue", (p, attributes) => attributes.Single().DefaultValue));

In the custom SqlGenerator

private void SetAnnotatedColumn(ColumnModel col)
{
    AnnotationValues values;
    if (col.Annotations.TryGetValue("SqlDefaultValue", out values))
    {
         col.DefaultValueSql = (string)values.NewValue;
    }
}

Then in the Migration Configuration constructor, register the custom SQL generator.

SetSqlGenerator("System.Data.SqlClient", new CustomMigrationSqlGenerator());

The above answers really helped, but only delivered part of the solution. The major issue is that as soon as you remove the Default value attribute, the constraint on the column in database won't be removed. So previous default value will still stay in the database.

Here is a full solution to the problem, including removal of SQL constraints on attribute removal. I am also re-using .NET Framework's native DefaultValue attribute.

Usage

[DatabaseGenerated(DatabaseGeneratedOption.Computed)]
[DefaultValue("getutcdate()")]
public DateTime CreatedOn { get; set; }

For this to work you need to update IdentityModels.cs and Configuration.cs files

IdentityModels.cs file

Add/update this method in your ApplicationDbContext class

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
            base.OnModelCreating(modelBuilder);
            var convention = new AttributeToColumnAnnotationConvention<DefaultValueAttribute, string>("SqlDefaultValue", (p, attributes) => attributes.SingleOrDefault().Value.ToString());
            modelBuilder.Conventions.Add(convention);
}

Configuration.cs file

Update your Configuration class constructor by registering custom Sql generator, like this:

internal sealed class Configuration : DbMigrationsConfiguration<ApplicationDbContext>
{
    public Configuration()
    {
        // DefaultValue Sql Generator
        SetSqlGenerator("System.Data.SqlClient", new DefaultValueSqlServerMigrationSqlGenerator());
    }
}

Next, add custom Sql generator class (you can add it to the Configuration.cs file or a separate file)

internal class DefaultValueSqlServerMigrationSqlGenerator : SqlServerMigrationSqlGenerator
{
    private int dropConstraintCount;

    protected override void Generate(AddColumnOperation addColumnOperation)
    {
        SetAnnotatedColumn(addColumnOperation.Column, addColumnOperation.Table);
        base.Generate(addColumnOperation);
    }

    protected override void Generate(AlterColumnOperation alterColumnOperation)
    {
        SetAnnotatedColumn(alterColumnOperation.Column, alterColumnOperation.Table);
        base.Generate(alterColumnOperation);
    }

    protected override void Generate(CreateTableOperation createTableOperation)
    {
        SetAnnotatedColumns(createTableOperation.Columns, createTableOperation.Name);
        base.Generate(createTableOperation);
    }

    protected override void Generate(AlterTableOperation alterTableOperation)
    {
        SetAnnotatedColumns(alterTableOperation.Columns, alterTableOperation.Name);
        base.Generate(alterTableOperation);
    }

    private void SetAnnotatedColumn(ColumnModel column, string tableName)
    {
        if (column.Annotations.TryGetValue("SqlDefaultValue", out var values))
        {
            if (values.NewValue == null)
            {
                column.DefaultValueSql = null;
                using var writer = Writer();

                // Drop Constraint
                writer.WriteLine(GetSqlDropConstraintQuery(tableName, column.Name));
                Statement(writer);
            }
            else
            {
                column.DefaultValueSql = (string)values.NewValue;
            }
        }
    }

    private void SetAnnotatedColumns(IEnumerable<ColumnModel> columns, string tableName)
    {
        foreach (var column in columns)
        {
            SetAnnotatedColumn(column, tableName);
        }
    }

    private string GetSqlDropConstraintQuery(string tableName, string columnName)
    {
        var tableNameSplitByDot = tableName.Split('.');
        var tableSchema = tableNameSplitByDot[0];
        var tablePureName = tableNameSplitByDot[1];

        var str = $@"DECLARE @var{dropConstraintCount} nvarchar(128)
SELECT @var{dropConstraintCount} = name
FROM sys.default_constraints
WHERE parent_object_id = object_id(N'{tableSchema}.[{tablePureName}]')
AND col_name(parent_object_id, parent_column_id) = '{columnName}';
IF @var{dropConstraintCount} IS NOT NULL
EXECUTE('ALTER TABLE {tableSchema}.[{tablePureName}] DROP CONSTRAINT [' + @var{dropConstraintCount} + ']')";

        dropConstraintCount++;
        return str;
    }
}

Your model properties don't have to be 'auto properties' Even though that is easier. And the DefaultValue attribute is really only informative metadata The answer accepted here is one alternative to the constructor approach.

public class Track
{

    private const int DEFAULT_LENGTH = 400;
    private int _length = DEFAULT_LENGTH;
    [DefaultValue(DEFAULT_LENGTH)]
    public int LengthInMeters {
        get { return _length; }
        set { _length = value; }
    }
}

vs.

public class Track
{
    public Track()
    {
        LengthInMeters = 400;   
    }

    public int LengthInMeters { get; set; }        
}

This will only work for applications creating and consuming data using this specific class. Usually this isn't a problem if data access code is centralized. To update the value across all applications you need to configure the datasource to set a default value. Devi's answer shows how it can be done using migrations, sql, or whatever language your data source speaks.