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Dynamic LINQ query to get Field value from Database

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耶瑟儿～
耶瑟儿～ 2021-01-18 03:20

is it possible?

Public String Get_Filed_By_Id(string table_Name,String Field_Name,string PK_val)
{
    string strRes=\"\";
    using(mydbcontext db=new mydbc
4条回答
  • 旧时难觅i
    旧时难觅i (楼主)
    2021-01-18 03:58

    Is it possible??

    Yes, it is.

    First, some helpers: 

    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, Expression>, IQueryable>>((source, selector) =>
            Queryable.Select(source, selector));
        private static readonly MethodInfo Where = GetGenericMethodDefinition<
            Func, Expression>, object>>((source, predicate) =>
            Queryable.Where(source, predicate));
        private static readonly MethodInfo FirstOrDefault = GetGenericMethodDefinition<
            Func, object>>(source =>
            Queryable.FirstOrDefault(source));
        private static MethodInfo GetGenericMethodDefinition(Expression e)
        {
            return ((MethodCallExpression)e.Body).Method.GetGenericMethodDefinition();
        }
        private static object Call(MethodInfo method, params object[] parameters)
        {
            return method.Invoke(null, parameters);
        }
    }
}



    and now your function:  
    


    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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