performing partial database update with dapper

Suppose that your User class is something like this one

public class User
{
    public int UserID { get; set; }
    public string Name {get; set;}
    public string FamilyName {get;set;}
    public int? Age { get; set; }
    public string Sex { get; set; }

}

(Note the int field defined as Nullable<int> to allow the insertion of a null value in the corresponding field)

Now, your code that sets the fields, reflecting a null value for your null properties, could be written simply as a normal update. All the stuff required to pass a null value as a parameter is done by the Dapper internals

// Initialize just two fields and leave the other to their defaults
// (null for both strings and nullable ints)
User u = new User();
u.UserID = 1;
u.Name = "Steve";
bool ok = UpdateUser(u);
if(ok) ......


public UpdateUser(User info)
{
    using(SqlConnection cnn = new SqlConnection(@"Data Source=(LOCAL);
                                                Initial Catalog=TestDB;
                                                Integrated Security=True;"))
    {
        cnn.Open();

        // Prepare the parameters to pass to Dapper Execute 
        var pms = new
        {
           UserID = info.UserID   
           FirstName = info.Name,
           FamilyName = info.FamilyName,  // <- this is null
           Age = info.Age,                // <- this is null
           Sex = info.Sex                 // <- this is null
        };

        int rows = cnn.Execute(@"UPDATE [UserTable] 
                                 SET FirstName= @FirstName,
                                     LastName = @LastName, 
                                     Age = @Age, 
                                     Sex = @Sex
                                 WHERE UserID = @UserID",
                                 pms);
         return rows != 0;
    }
}