c# - Reading from Datagrid Textbox -

i have datagrid , in there 3 columns , values of 2 columns fetched database , populated whereas third column textbox user enters value. have save value database . facing problem here . when enter value in ui , showing null in background code ..what doing wrong ?

   <datagrid autogeneratecolumns="false" height="233" name="datagrid1" width="360" >     <datagrid.columns>       <datagridtextcolumn header="x" binding="{binding path=x}" width="120" isreadonly="true" />    <datagridtextcolumn header="y" binding="{binding path=y}" width="120"  isreadonly="true"/>     <datagridtextcolumn header="value/ref" width="120" binding="{binding path=z, mode=twoway}"  isreadonly="false" />       </datagrid.columns> 

i have class containing variables x,y,z . creating list of class , trying add values class

with xaml code , simple code snippet, behaviour want (what understand):

class     {         private string _x, _y, _z;          public string x { { return _x; } set { _x = value; } }         public string y { { return _y; } set { _y = value; } }         public string z { { return _z; } set { _z = value; } }     }  public partial class mainwindow : window     {         private list<a> mydata;          public mainwindow()         {             initializecomponent();              mydata = new list<a>();             mydata.add(new a() { x = "x1", y = "y1" });             mydata.add(new a() { x = "x2", y = "y2" });             mydata.add(new a() { x = "x3", y = "y3" });              datagrid1.itemssource = mydata;         }     } 

if type in data in column value/ref, list behind updated.