In EF Code First is there a data annotation for description field?

问题

While designing a table in MS-SQL you have a choice to add a description for each column you add to the table. Is it possible in EF Code First to do same with help of Data Annotations?

回答1:

Before you are using my soultion please read a little bit about:

Extended Properties:
https://technet.microsoft.com/en-us/library/ms190243(v=sql.105).aspx

Attributes:
http://www.dotnetperls.com/attribute

My solution will allow you to do a shadow description for any property like that:

[Description("My Column description!")]
public string RegionCity { get; set; }

Notes:

Copy paste the soultion and everything should works (donnot forget the app.config connection string)!
You can use the sql command IF NOT EXISTS to check if the extended property already exist or not.

You can build your own generic logic or C# extension to read the descirption from any property this should be easy a just have added a an example with string.

using System;
using System.Linq;

namespace EntityFrameworkDemo
{
 using System.Data.Entity;

public class Program
{
    public static void Main()
    {
        Database.SetInitializer(new DropCreateDatabaseAlways<MyDbContext>());


        using (var myDbContext = new MyDbContext("DefaultConnection"))
        {
            // Maybe you do not need this line!
            myDbContext.Database.Initialize(true);

            // Adding one time or use the IF NOT EXISTS SQL Command!
            var c = new ColumnsDescription();
            c.AddColumnsDescriptions(myDbContext);

            var region = new Region { RegionCity = "Test 1", RegionSeconcdCity = "Test2" };
            myDbContext.Regions.Add(region);
            myDbContext.SaveChanges();
        }


        // Reading the extended properties
        using (var myDbContext = new MyDbContext("DefaultConnection"))
        {
            var ep = "select value from fn_listextendedproperty('MS_Description','schema','dbo','table', 'Regions', 'column', 'RegionCity');";

            // For example you can read your extend property like following or you make a generic reader
            var properties = myDbContext.Database.SqlQuery<string>(ep).First();

            Console.WriteLine(properties);
        }
    }

    [AttributeUsage(AttributeTargets.Property)]
    public class DescriptionAttribute : Attribute
    {
        string value;

        public DescriptionAttribute(string id)
        {
            this.value = id;
        }

        public string Value
        {
            get { return this.value; }
        }
    }

    public class ColumnsDescription
    {
        public void AddColumnsDescriptions(DbContext mydbContext)
        {
            // Fetch all the DbContext class public properties which contains your attributes
            var dbContextProperies = typeof(DbContext).GetProperties().Select(pi => pi.Name).ToList();

            // Loop each DbSets of type T
            foreach (var item in typeof(MyDbContext).GetProperties()
                .Where(p => dbContextProperies.IndexOf(p.Name) < 0)
                .Select(p => p))
            {
                if (!item.PropertyType.GetGenericArguments().Any())
                {
                    continue;
                }

                // Fetch the type of "T"
                var entityModelType = item.PropertyType.GetGenericArguments()[0];
                var descriptionInfos = from prop in entityModelType.GetProperties()
                                       where prop.GetCustomAttributes(typeof(DescriptionAttribute), true).Any()
                                       select new { ColumnName = prop.Name, Attributes = prop.CustomAttributes };

                foreach (var descriptionInfo in descriptionInfos)
                {
                    // Sql to create the description column and adding 
                    var addDiscriptionColumnSql =
                        @"sp_addextendedproperty  @name = N'MS_Description', @value = '"
                        + descriptionInfo.Attributes.First().ConstructorArguments.First()
                        + @"', @level0type = N'Schema', @level0name = dbo,  @level1type = N'Table',  @level1name = "
                        + entityModelType.Name + "s" + ", @level2type = N'Column', @level2name ="
                        + descriptionInfo.ColumnName;

                    var sqlCommandResult = mydbContext.Database.ExecuteSqlCommand(addDiscriptionColumnSql);
                }
            }
        }
    }

    public class Region
    {
        public int Id { get; set; }

        [Description("My Column description!")]
        public string RegionCity { get; set; }

        [Description("My Second Column description!")]
        public string RegionSeconcdCity { get; set; }

    }

    public class MyDbContext : DbContext
    {
        public DbSet<Region> Regions { get; set; }

        public MyDbContext(string connectionString)
            : base("name=" + connectionString)
        {
        }
    }
}
}

来源：https://stackoverflow.com/questions/38996810/in-ef-code-first-is-there-a-data-annotation-for-description-field

标签

sql-server

entity-framework

ef-code-first

data-annotations