Question

有可能吗？

Public String Get_Filed_By_Id(string table_Name,String Field_Name,string PK_val)
{
    string strRes="";
    using(mydbcontext db=new mydbcontext())
    {
      var x=db.table_Name.Where(p=>p.Id=PK_val).FirstOrDefault().Field_Name;
      strRes=Convert.Tostring(x);
    }
 return strRes;
}

OR

var x=(from o in db.table_Name where o.Id=PK_val select o.Field_Name).FirstOrDefault();

在这里，我正在传递Table_Name，Column_Name和条件值（PK_val）以在特定条件下从Column_Name获取Table_Name（ Id=Pk_val）。

有可能吗？

Answer 1

有可能吗？

是的，确实如此。

首先，一些帮手：

html, body{
  height: 100%;
}

#app{
  height: 100%;
}

.board{
  background: silver;
  width: 100%;
  height: 100%;
}

.column{
  display: inline-block;
  margin: 0 0.5em;
  background: gray;
  width: 20%;
  height: 100%;
}

.task{
  background: yellow;
  width: 90%;
  margin: auto;
  padding: 0.2em;
  text-align: center;
}

现在你的功能：

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace YourNamespace
{
    internal static class DbHelpers
    {
        public static object GetColumnById(this object dbContext, string tableName, string columnName, object id)
        {
            var table = (IQueryable)dbContext.GetType().GetProperty(tableName).GetValue(dbContext, null);
            var row = Expression.Parameter(table.ElementType, "row");
            var filter = Expression.Lambda(Expression.Equal(Expression.Property(row, "Id"), Expression.Constant(id)), row);
            var column = Expression.Property(row, columnName);
            var selector = Expression.Lambda(column, row);
            var query = Call(Where.MakeGenericMethod(row.Type), table, filter);
            query = Call(Select.MakeGenericMethod(row.Type, column.Type), query, selector);
            var value = Call(FirstOrDefault.MakeGenericMethod(column.Type), query);
            return value;
        }
        private static readonly MethodInfo Select = GetGenericMethodDefinition<
            Func<IQueryable<object>, Expression<Func<object, object>>, IQueryable<object>>>((source, selector) =>
            Queryable.Select(source, selector));
        private static readonly MethodInfo Where = GetGenericMethodDefinition<
            Func<IQueryable<object>, Expression<Func<object, bool>>, object>>((source, predicate) =>
            Queryable.Where(source, predicate));
        private static readonly MethodInfo FirstOrDefault = GetGenericMethodDefinition<
            Func<IQueryable<object>, object>>(source =>
            Queryable.FirstOrDefault(source));
        private static MethodInfo GetGenericMethodDefinition<TDelegate>(Expression<TDelegate> e)
        {
            return ((MethodCallExpression)e.Body).Method.GetGenericMethodDefinition();
        }
        private static object Call(MethodInfo method, params object[] parameters)
        {
            return method.Invoke(null, parameters);
        }
    }
}

Answer 2

我也有这个问题，我知道这不是你想要的，你需要编写更多的代码，但它比你想写的更干净。
使用存储库模式
对于每个表，您应该有一个模型类和Repository类考虑一下这段代码（这个代码来自我的一个项目）
这是我的评论表（这可以是带或不带导航属性的任何内容）

  public sealed class Comment
{
    public string CommentText { get; set; }
    public DateTime PostDate { get; set; }
    public int PostId { get; set; }
    public int? PageId { get; set; }
    public Page Page { get; set; }
    public User User { get; set; }
    public string UserId { get; set; }
    public int? ParentId { get; set; }
    public Comment[] ChildComments { get; set; }
}

RepositoryComment

  public sealed class CommentRepository : BaseRepository<Comment>
{
    public CommentRepository(BabySitterContext context)
        : base(context)
    {
    }
}

以及使用表名（此处为模型）和字段发送查询的基类（可以扩展clas以获得更多功能）

public class BaseRepository<T> where T : class
{
    protected BabySitterContext Context;
    private readonly PluralizationService _pluralizer = PluralizationService.CreateService(CultureInfo.GetCultureInfo("en"));
    public BaseRepository(BabySitterContext context)
    {
        this.Context = context;
    }
    public bool Add(T t)
    {
        Context.Set<T>().Add(t);
        Context.SaveChanges();
        return true;
    }
    public bool Update(T t)
    {
        var entityName = GetEntityName<T>();

        object originalItem;
        var key = ((IObjectContextAdapter)Context).ObjectContext.CreateEntityKey(entityName, t);
        if (((IObjectContextAdapter)Context).ObjectContext.TryGetObjectByKey(key, out originalItem))
        {
            ((IObjectContextAdapter)Context).ObjectContext.ApplyCurrentValues(key.EntitySetName, t);
        }
        Context.SaveChanges();
        return true;
    }
    public void Attach(T t)
    {
        if (t == null)
        {
            throw new ArgumentNullException("t");
        }

        Context.Set<T>().Attach(t);
        Context.SaveChanges();
    }
    public void Remove(T t)
    {
        if (t == null)
        {
            throw new ArgumentNullException("t");
        }
        Context.Set<T>().Remove(t);
        Context.SaveChanges();
    }
    public IEnumerable<T> Get(Expression<Func<T, bool>> filter = null, Func<IQueryable<T>, IOrderedQueryable<T>> orderBy = null, string includeProperties = "")
    {
        IQueryable<T> query = Context.Set<T>();

        if (filter != null)
        {
            query = query.Where(filter.Expand());
        }

        foreach (var includeProperty in includeProperties.Split
            (new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries))
        {
            query = query.Include(includeProperty);
        }

        if (orderBy != null)
        {
            return orderBy(query).ToList();
        }
        else
        {
            return query.ToList();
        }
    }
    private string GetEntityName<TEntity>() where TEntity : class
    {
        return string.Format("{0}.{1}", ((IObjectContextAdapter)Context).ObjectContext.DefaultContainerName, _pluralizer.Pluralize(typeof(TEntity).Name));

    }

    public virtual IEnumerable<T> GetByBusinessKey(T entity)
    {
        return null;
    }
}

对于任何其他表，只需创建模型类和reposiotry然后从基类继承

使用代码

 var context = new BabySitterContext();
 var  _commentRepository = new CommentRepository(context);
 var comment = _commentRepository.Get(x => x.PostId == id).FirstOrDefault();

Answer 3

不，但是这样

Hello, World!
Overridden property.
Overridden function.

动态LINQ查询从数据库获取Field值

3 个答案: