动态LINQ查询以从数据库获取字段值 [英] Dynamic LINQ query to get Field value from Database
本文介绍了动态LINQ查询以从数据库获取字段值的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
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,我要传递Table_Name
,Column_Name
和条件值(PK_val
)以在特定条件(Id=Pk_val
)内从Table_Name
获取Column_Name
.
Here, i'm passing Table_Name
,Column_Name
and the Condition value(PK_val
) to Get the Column_Name
from Table_Name
within a Certain Condition(Id=Pk_val
).
有可能吗?
推荐答案
有可能吗?
是的.
首先,一些助手:
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);
}
}
}
现在是您的功能:
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));
}
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