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 I'm leaving a note for others. I achieved what was needed with an attribute and decorated my model class fields with that attribute as I wanted.

[SqlDefaultValue("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 SqlDefaultValueAttribute(string defaultValue)
    {
        DefaultValue = defaultValue;
    }

    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.

What I did, I initialized values in the constructor of the entity

Note: DefaultValue attributes won't set the values of your properties automatically, you have to do it yourself

After @SedatKapanoglu comment, I am adding all my approach that works, because he was right, just using the fluent API does not work.

1- Create custom code generator and override Generate for a ColumnModel.

   public class ExtendedMigrationCodeGenerator : CSharpMigrationCodeGenerator
{

    protected override void Generate(ColumnModel column, IndentedTextWriter writer, bool emitName = false)
    {

        if (column.Annotations.Keys.Contains("Default"))
        {
            var value = Convert.ChangeType(column.Annotations["Default"].NewValue, column.ClrDefaultValue.GetType());
            column.DefaultValue = value;
        }


        base.Generate(column, writer, emitName);
    }

}

2- Assign the new code generator:

public sealed class Configuration : DbMigrationsConfiguration<Data.Context.EfSqlDbContext>
{
    public Configuration()
    {
        CodeGenerator = new ExtendedMigrationCodeGenerator();
        AutomaticMigrationsEnabled = false;
    }
}

3- Use fluent api to created the Annotation:

public static void Configure(DbModelBuilder builder){    
builder.Entity<Company>().Property(c => c.Status).HasColumnAnnotation("Default", 0);            
}

I admit that my approach escapes the whole "Code First" concept. But if you have the ability to just change the default value in the table itself... it's much simpler than the lengths that you have to go through above... I'm just too lazy to do all that work!

It almost seems as if the posters original idea would work:

[DefaultValue(true)]
public bool IsAdmin { get; set; }

I thought they just made the mistake of adding quotes... but alas no such intuitiveness. The other suggestions were just too much for me (granted I have the privileges needed to go into the table and make the changes... where not every developer will in every situation). In the end I just did it the old fashioned way. I set the default value in the SQL Server table... I mean really, enough already! NOTE: I further tested doing an add-migration and update-database and the changes stuck.

In .NET Core 3.1 you can do the following in the model class:

    public bool? Active { get; set; } 

In the DbContext OnModelCreating you add the default value.

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Foundation>()
            .Property(b => b.Active)
            .HasDefaultValueSql("1");

        base.OnModelCreating(modelBuilder);
    }

Resulting in the following in the database

Note: If you don't have nullable (bool?) for you property you will get the following warning

The 'bool' property 'Active' on entity type 'Foundation' is configured with a database-generated default. This default will always be used for inserts when the property has the value 'false', since this is the CLR default for the 'bool' type. Consider using the nullable 'bool?' type instead so that the default will only be used for inserts when the property value is 'null'.

It's simple! Just annotate with required.

[Required]
public bool MyField { get; set; }

the resultant migration will be:

migrationBuilder.AddColumn<bool>(
name: "MyField",
table: "MyTable",
nullable: false,
defaultValue: false);

If you want true, change the defaultValue to true in the migration before updating the database

I found that just using Auto-Property Initializer on entity property is enough to get the job done.

For example:

public class Thing {
    public bool IsBigThing{ get; set; } = false;
}

In EF core released 27th June 2016 you can use fluent API for setting default value. Go to ApplicationDbContext class, find/create the method name OnModelCreating and add the following fluent API.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<YourTableName>()
        .Property(b => b.Active)
        .HasDefaultValue(true);
}

If you are using Entity Framework Core and want to use Data Annotations instead of Fluent API, you will need to create a custom attribute along with a custom convention.

SqlDefaultValueAttribute.cs

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

    public string DefaultValue { get; set; }
}

SqlDefaultValueConvention.cs

public class SqlDefaultValueConvention
    : PropertyAttributeConventionBase<SqlDefaultValueAttribute>
{
    public SqlDefaultValueConvention(ProviderConventionSetBuilderDependencies dependencies)
        : base(dependencies) { }

    protected override void ProcessPropertyAdded(
        IConventionPropertyBuilder propertyBuilder,
        SqlDefaultValueAttribute attribute,
        MemberInfo clrMember,
        IConventionContext context)
    {
        propertyBuilder.HasDefaultValueSql(attribute.DefaultValue);
    }
}

You will then need to register the convention within your DbContext

public class ApplicationDbContext : DbContext
{
    protected override void ConfigureConventions(
        ModelConfigurationBuilder configurationBuilder)
    {
        configurationBuilder.Conventions.Add(services =>
            new SqlDefaultValueConvention(
                services.GetRequiredService<ProviderConventionSetBuilderDependencies>()));
    }
}

You can now use it like this:

public class MyEntity
{
    [SqlDefaultValue("getdate()")]
    public DateTime CreatedAt { get; set; }
}

using System.ComponentModel;

[DefaultValue(true)]

public bool Active { get; set; }

Just Overload the default constructor of Model class and pass any relevant parameter which you may or may not use. By this you can easily supply default values for attributes. Below is an example.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Aim.Data.Domain
{
    [MetadataType(typeof(LoginModel))]
    public partial class Login
    {       
        public Login(bool status)
        {
            this.CreatedDate = DateTime.Now;
            this.ModifiedDate = DateTime.Now;
            this.Culture = "EN-US";
            this.IsDefaultPassword = status;
            this.IsActive = status;
            this.LoginLogs = new HashSet<LoginLog>();
            this.LoginLogHistories = new HashSet<LoginLogHistory>();
        }


    }

    public class LoginModel
    {

        [Key]
        [ScaffoldColumn(false)] 
        public int Id { get; set; }
        [Required]
        public string LoginCode { get; set; }
        [Required]
        public string Password { get; set; }
        public string LastPassword { get; set; }     
        public int UserGroupId { get; set; }
        public int FalseAttempt { get; set; }
        public bool IsLocked { get; set; }
        public int CreatedBy { get; set; }       
        public System.DateTime CreatedDate { get; set; }
        public Nullable<int> ModifiedBy { get; set; }      
        public Nullable<System.DateTime> ModifiedDate { get; set; }       
        public string Culture { get; set; }        
        public virtual ICollection<LoginLog> LoginLogs { get; set; }
        public virtual ICollection<LoginLogHistory> LoginLogHistories { get; set; }
    }

}

I tried the [DefaultValue(getdate())] and [DefaultValue(getutcdate())] attributes but this did not work, if the column in the database is not null.

I used a constructor to set the default value for the datetime fields and it worked fine when creating new records using EF:

public MyModel
{
    DateTime currDateTime = DateTime.Now;
    CreationDT = currDateTime;
}

The answer provided by J-M works well for me but with some modification, as I am using Npgsql and the Generate function will not get run during Update-Database as the provider is different (the original answer is for SqlServer).

If you are using Npgsql as your provider, just make some slight modification so it works with Npgsql:

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("Npgsql", new DefaultValueNpgsqlMigrationSqlGenerator());
    }
}

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

internal class DefaultValueNpgsqlMigrationSqlGenerator : NpgsqlMigrationSqlGenerator
{
    private int dropConstraintCount;

    public override IEnumerable<MigrationStatement> Generate(IEnumerable<MigrationOperation> migrationOperations, string providerManifestToken)
    {
        foreach (var operation in migrationOperations)
        {
            if (operation is AddColumnOperation addColumnOperation)
            {
                SetAnnotatedColumn(addColumnOperation.Column, addColumnOperation.Table);
            }
            else if (operation is AlterColumnOperation alterColumnOperation)
            {
                SetAnnotatedColumn(alterColumnOperation.Column, alterColumnOperation.Table);
            }
            else if (operation is CreateTableOperation createTableOperation)
            {
                SetAnnotatedColumns(createTableOperation.Columns, createTableOperation.Name);
            }
            else if (operation is AlterTableOperation alterTableOperation)
            {
                SetAnnotatedColumns(alterTableOperation.Columns, alterTableOperation.Name);
            }
        }

        return base.Generate(migrationOperations, providerManifestToken);
    }

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

                // Drop Constraint
                var operations = new List<MigrationOperation>
            {
                new SqlOperation(GetSqlDropConstraintQuery(tableName, column.Name))
            };
                Convert(operations);
            }
            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;
    }
} 

Even from .NET Core 1.0, It is possible to set default values when you are using the code first approach. See the following code snippet.

using System.ComponentModel;

private bool _myVal = false;

[DefaultValue(true)]
public bool Active
{
    get
    {
        return _myVal;
    }
    set
    {
        _myVal = value;
    }
}

Read for more: Microsoft official docs

The Entity Framework Core Fluent API HasDefaultValue method is used to specify the default value for a database column mapped to a property. The value must be a constant.

public class Contact
{
    public int ContactId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public bool IsActive { get; set; }
    public DateTime DateCreated { get; set; }
}
public clas SampleContext : DbContext
{
    public DbSet<Contact> Contacts { get; set; }
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Context>()
            .Propery(p => p.IsActive)
            .HasDefaultValue(true);
    }
}

Or

like it!

You can also specify a SQL fragment that is used to calculate the default value:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Blog>()
        .Property(b => b.Created)
        .HasDefaultValueSql("getdate()");
}