动态LINQ查询从数据库获取Field值

Question

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;
}

要么

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并且条件值(PK_val)Column_Name从Table_Name某个条件(Id=Pk_val)中获取.

可能吗？？

Answer 1

可能吗？？

是的.

首先,一些帮手:

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);
        }
    }
}

现在你的功能:

public string Get_Field_By_Id(string table_Name, string field_Name, string PK_val)
{
    using (var db = new mydbcontext())
        return Convert.ToString(db.GetColumnById(table_Name, field_Name, PK_val));
}