可为空的属性到实体字段，实体框架通过代码优先

Just omit the [Required] attribute from the string somefield property. This will make it create a NULLable column in the db.

To make int types allow NULLs in the database, they must be declared as nullable ints in the model:

// an int can never be null, so it will be created as NOT NULL in db
public int someintfield { get; set; }


// to have a nullable int, you need to declare it as an int?
// or as a System.Nullable<int>
public int? somenullableintfield { get; set; }
public System.Nullable<int> someothernullableintfield { get; set; }

The other option is to tell EF to allow the column to be null:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<SomeObject>().Property(m => m.somefield).IsOptional();
base.OnModelCreating(modelBuilder);
}

This code should be in the object that inherits from DbContext.

Jon's answer didn't work for me as I got a compiler error CS0453 C# The type must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method

This worked for me though:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<SomeObject>().HasOptional(m => m.somefield);
base.OnModelCreating(modelBuilder);
}

In Ef .net core there are two options that you can do; first with data annotations:

public class Blog
{
public int BlogId { get; set; } // optinal case


[Required]
public string Url { get; set; } // required case
}

Or with fluent api:

class MyContext : DbContext
{
public DbSet<Blog> Blogs { get; set; }


protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Blog>()
.Property(b => b.Url)
.IsRequired(false)//optinal case
.IsRequired()//required case
;
}
}


public class Blog
{
public int BlogId { get; set; }
public string Url { get; set; }
}

There are more details here