在EF Code First中有一个描述字段的数据注释？ [英] In EF Code First is there a data annotation for description field?

问题描述

在 MS-SQL 中设计表时，您可以选择为每个列添加一个描述到桌子 EF Code First 可以在数据注释的帮助下执行相同操作？

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?

扩展属性

解决方案

推荐答案

在使用我的soultion之前，请阅读一下：

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

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

属性：

http://www.dotnetperls.com/attribute

我的解决方案将允许您为这样的任何属性进行阴影描述：

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

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

注意：

• 复制粘贴soultion，一切都应该有效（不要忘记app.config连接字符串）！


• 可以使用sql命令IF NOT EXISTS检查扩展属性是否已经存在。



• 您可以构建您自己的通用逻辑或C＃扩展，以便从任何属性读取描述，这应该是简单的添加一个使用字符串的例子。

• 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)
        {
        }
    }
}
}   

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