Silverlight slider control "object reference not set to object instance" error Solution

After the value is set in the XAML file and the solution is compiled, an error "object reference is not set to the instance of the object" is reported"

Scolour. XAML FileCode

Code

  <  Usercontrol  X: Class  = "Sample. scolour"  
Xmlns  = "Http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
Xmlns: x  = "Http://schemas.microsoft.com/winfx/2006/xaml" 
Xmlns: d  = "Http://schemas.microsoft.com/expression/blend/2008"  
Xmlns: MC  = "Http://schemas.openxmlformats.org/markup-compatibility/2006"  
MC: ignorable  = "D"  
D: designheight  = "300"  D: designwidth  = "400"  >  
  <  Grid  X: Name = "Layoutroot"  Background  = "White"  >  
  <  Grid. rowdefinitions  >  
  <  Rowdefinition  Height  = "120"  />  
  <  Rowdefinition Height  = "120"  />  
  </  Grid. rowdefinitions  >  
  <  Grid. columndefinitions  >  
  <  Columndefinition  Width  = "200"  /> 
  <  Columndefinition  Width  = "*"  />  
  </  Grid. columndefinitions  >  
  <  Rectangle  Grid. Row  = "0"  Grid. Column  = "1" X: Name  = "Previewcolour"  Fill  = "# Ffff6600"  Margin  = "15"     > </  Rectangle  >  
  <  Stackpanel  Grid. Row  = "1"  Grid. Column  = "1" >  
  <  Textblock  Fontsize  = "12"  >  Color  </  Textblock  >  
  <  Textbox  X: Name  = "Hexcolour"  Text = "# Ffff6600"  Height  = "30"  Width  = "160"  Margin  = "5"  Horizontalalignment  = "Left"  Fontsize  = "11"  > </  Textbox  >  
  </  Stackpanel >  
  <  Stackpanel  Grid. Row  = "0"  Grid. Column  = "0"  Grid. rowspan  = "2"  Verticalalignment  = "Center"  >  
  <  Textblock  Text = "Alpha"  Fontsize  = "11"  Margin  = "10, 15, 0, 0"  />  
  <  Slider  X: Name  = "Slideralpha"  Margin  = "20, 0, 10, 0"  Maximum  = "255"  Value  = "255" Valuechanged  = "Slider_valuechanged"  />  
  <  Textblock  Text  = "Red"  Fontsize  = "11"  Margin  = "10, 15, 0, 0"  />  
  <  Slider  X: Name = "Sliderred"  Margin  = "20, 0, 10, 0"  Maximum  = "255"  Value  = "255"  Valuechanged  = "Slider_valuechanged"  />  
  <  Textblock  Text  = "Green"  Fontsize = "11"  Margin  = "10, 15, 0, 0"  />  
  <  Slider  X: Name  = "Slidergreen"  Margin  = "20, 0, 10, 0"  Maximum  = "255"  Value  = "102"  Valuechanged = "Slider_valuechanged"  />  
  <  Textblock  Text  = "Blue"  Fontsize  = "11"  Margin  = "10, 15, 0, 0"  />  
  <  Slider  X: Name  = "Sliderblue" Margin  = "20, 0, 10, 0"  Maximum  = "255"  Value  = "0"  Valuechanged  = "Slider_valuechanged"  />  
  </  Stackpanel  >  
  </  Grid  > 
  </  Usercontrol  > 

 

 

Scolour. XAML. CS File Code

 

Code

  1   Using  System. Windows. documents;
  2      Using System. Windows. input;
  3      Using  System. Windows. Media;
  4      Using  System. Windows. Media. animation;
  5      Using  System. Windows. shapes;
  6   Namespace  Sample
 7   {
  8   Public     Partial     Class  Scolour: usercontrol
  9   {
  10   Public  Scolour ()
  11   {
  12  Initializecomponent ();
  13   }
  14   Private     Void  Slider_valuechanged (  Object  Sender, routedpropertychangedeventargs  <  Double  >  E)
  15   {
 16   Color mycolor  =  Color. fromargb ((  Byte  ) Slideralpha. Value ,(  Byte  ) Sliderred. Value ,(  Byte  ) Slidergreen. Value ,(  Byte  ) Sliderblue. value );
  17   Previewcolour. Fill  =    New  Solidcolorbrush (mycolor );
  18   Hexcolour. Text  =  Mycolor. tostring ();
  19   }
  20   }
  21   } 

 

Check the code carefully. It seems that there are no errors, but the system always prompts "the object is not referenced to the instance of the object ". Later, after checking the information, this may be a bug in Silverlight. If valuechanged is defined, it seems that the value attribute of Slider cannot be assigned an initial value in XAML. We can assign values to the scolour constructor:

 

 

 1 Public  Scolour ()
  2  {
  3  Initializecomponent ();
  4  This  . Slideralpha. Value  =     255  ;
  5  This  . Sliderred. Value  =    255  ;
  6  This  . Slidergreen. Value  =     102  ;
  7  This  . Sliderblue. Value  =     0  ;
  8 } 

 

Running interface, a simple Color Filter