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How to force WPF textboxes contents to be displayed above other controls similar to excel cells

I have an ItemsControl that contains a textbox for each binding item and I want to allow overlaying text box content if its content is wider than the textbox width similar to excel cells overlaying behavior.
Is there a way to do this?
<ItemsControl ItemsSource="{Binding Path=MyCollection}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal" Width="100"/>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding}" TextWrapping="WrapWithOverflow"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>

You can use tooltip for that. You just need to bind the source with itself.
<ListView ItemsSource="{Binding Path=MyStringCollection}">
<ListView.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal" Width="100"/>
</ItemsPanelTemplate>
</ListView.ItemsPanel>
<ListView.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding}" TextWrapping="WrapWithOverflow" ToolTip="{Binding RelativeSource={RelativeSource Self}, Path=Text}"/>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
For further information:
DrawToolTip Event
Set ToolTipSize

To replicate a similar behavior, you can make use of a Popup.
To implement a similar behavior, you first must disable content wrapping of your TextBox.
Then replace the current TextBox with a TextBlock, which is used to display the text.
The Popup, which actually contains the editable TextBox, will then overlay this TextBlock at the exact position, thus hiding the TextBlock to make it appear to stretch and overlay the adjacent items.
A MultiTrigger will close the Popup as soon as the focus moved outside the ListBoxItem.
To improve performance you should use the VirtualizingStackPanel as items host.
<ListView ItemsSource="{Binding MyStringCollection}"
HorizontalAlignment="Left"
Width="800">
<ListView.ItemsPanel>
<ItemsPanelTemplate>
<VirtualizingStackPanel Orientation="Horizontal" />
</ItemsPanelTemplate>
</ListView.ItemsPanel>
<ListView.ItemTemplate>
<DataTemplate>
<Grid>
<Border BorderThickness="1"
BorderBrush="Gray">
<TextBlock Text="{Binding}" />
</Border>
<Popup x:Name="EditableTextSiteHost"
PlacementTarget="{Binding ElementName=TextSite}"
Placement="Relative"
Height="{Binding ElementName=TextSite, Path=ActualHeight}"
AllowsTransparency="True"
FocusManager.FocusedElement="{Binding ElementName=EditableTextSite}">
<TextBox x:Name="EditableTextSite"
Text="{Binding TextData}" />
</Popup>
</Grid>
<DataTemplate.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding RelativeSource={RelativeSource AncestorType=ListBoxItem}, Path=IsSelected}"
Value="True" />
<Condition Binding="{Binding RelativeSource={RelativeSource AncestorType=ListBoxItem}, Path=IsKeyboardFocusWithin}"
Value="True" />
</MultiDataTrigger.Conditions>
<Setter TargetName="EditableTextSiteHost"
Property="IsOpen"
Value="True" />
</MultiDataTrigger>
</DataTemplate.Triggers>
</DataTemplate>
</ListView.ItemTemplate>
<ListView.ItemContainerStyle>
<Style TargetType="ListBoxItem">
<!-- Make items overlap -->
<Setter Property="Margin"
Value="-2,0,0,0" />
<Setter Property="Padding"
Value="0" />
<Setter Property="Width"
Value="50" />
<Style.Triggers>
<!-- Apply zero Margin on the first item -->
<DataTrigger Binding="{Binding RelativeSource={RelativeSource PreviousData}}"
Value="{x:Null}">
<Setter Property="Margin"
Value="0,0,0,0" />
</DataTrigger>
</Style.Triggers>
</Style>
</ListView.ItemContainerStyle>
</ListView>
To-do
This is just a raw example, a proof of concept. You would have to improve the behavior. For example, you would want to close the Popup when the user scrolls or moves the Window. Otherwise, since Popup itself is a Window, the Popup would not move to follow the placement target. You could move the related logic to an attached behavior.
You likely also want to improve the selection behavior. Currently the selection highlight border does not (virtually) extend to surround the Popup. You have to mimic this by applying a Border on the TextBox that will replicate the ListBoxItem highlight border.

I managed to produce the excel cells overlay behavior by using a Grid with dynamic column count using this helper dependency properties https://rachel53461.wordpress.com/2011/09/17/wpf-grids-rowcolumn-count-properties/ as a container template of ItemsControl and binding the column index of each textbox to the ordered item index and binding Grid.ZIndex to the reversed index to be displayed above the adjacent text boxes.
<ItemsControl ItemsSource="{Binding MyCollection}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Grid HorizontalAlignment="Left" helpers:GridHelpers.ColumnCount="{Binding MyCollection.Count}"/>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemContainerStyle>
<Style>
<Setter Property="Grid.Column" Value="{Binding ItemIndex}"/>
<Setter Property="Grid.ZIndex" Value="{Binding ReversedIndex}" />
<Setter Property="Grid.ColumnSpan" Value="{Binding MaxMergedCells}" />
</Style>
</ItemsControl.ItemContainerStyle>
<ItemsControl.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal" HorizontalAlignment="Left">
<TextBox MinWidth="30" Text="{Binding }"/>
</StackPanel>
</DataTemplate>
</ItemsControl.ItemTemplate>

Bind Collection of User Control's to WPF Grid [duplicate]

I am using a Grid as ItemsPanel for a list dynamically bound to an ItemsControl. The code below is working - with a remaining problem: I can’t find a way to dynamically initialize the ColumnDefinitions and RowDefinitions of the grid. As consequence all values are placed on top of each other.
<ItemsControl ItemsSource="{Binding Cells}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Grid/>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemContainerStyle>
<Style>
<Setter Property="Grid.Row" Value="{Binding RowIndex}"/>
<Setter Property="Grid.Column" Value="{Binding ColumnIndex}"/>
</Style>
</ItemsControl.ItemContainerStyle>
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Value}" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
Please be aware, that I am searching an answer according the MVVM pattern. Therefore sub classing and code behind are just workarounds, but no solutions.

You'll need some way to tell the Grid how many Rows/Columns it has. Perhaps as each Item loads you could check the value of RowIndex and ColumnIndex and add Rows/Columns to the Grid if needed.
As another alternative, perhaps you can expose RowCount and ColumnCount properties in your ViewModel that return the max RowIndex and ColumnIndex, and in the Grid's Loaded event add however many Columns/Rows you need.
I find it perfectly acceptable to use code-behind in MVVM IF the code is related to the UI only.
Another idea would be to arrange your items in your code-behind into a 2D grid before returning it to the View, and then bind that Grid to a DataGrid with AutoGenerateColumns=True and the headers removed
Update
My current solution to this problem is to use a set of AttachedProperties for a Grid that allow you to bind RowCount and ColumnCount properties to a property on the ViewModel
You can find the code for my version of the attached properties on my blog here, and they can be used like this:
<ItemsPanelTemplate>
<Grid local:GridHelpers.RowCount="{Binding RowCount}"
local:GridHelpers.ColumnCount="{Binding ColumnCount}" />
</ItemsPanelTemplate>

Your grid has zero rows and columns so everything will be displayed on top of each other. Do something like below and it will work.
<ItemsControl ItemsSource="{Binding Cells}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
</Grid>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemContainerStyle>
<Style>
<Setter Property="Grid.Row" Value="{Binding RowIndex}" />
<Setter Property="Grid.Column" Value="{Binding ColumnIndex}" />
</Style>
</ItemsControl.ItemContainerStyle>
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Value}" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>

i found this from here The ItemsControl
<ItemsControl ItemsSource="{Binding VehicleMakes}" >
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<UniformGrid Columns="3" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Button Width="300" Height="100" Click="ButtonOption_Click" Tag="{Binding Name}">
<StackPanel Orientation="Vertical">
<Image
Initialized="Image_Initialized"
Tag="{Binding ResourseImageName}"
Width="116"
Height="30"
Margin="0,0,0,10" >
</Image>
<TextBlock Text="{Binding Name}" VerticalAlignment="Bottom" HorizontalAlignment="Center"/>
</StackPanel>
</Button>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>

ListBox.ItemTemplate with long text does not match ListBox width

I'm trying to put a StackPanel inside a ListBox and this inside a Grid that is a part of a UserControl.
So far I have come up with the following XAML but I cannot fix the width. When you take a look at the screenshot you'll see that the StackPanel is few pixels too wide. I tried HorizontalAlignment=Stretch on every element but on the Width={Binding...} manages to limt the width, although not perfectly. What do I miss?
<UserControl x:Class="Reusable.Apps.ExceptionControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:Reusable.Apps"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800" Width="{Binding (Window.Width), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}">
<UserControl.Resources>
<local:ExceptionVisualizerData x:Key="DesignViewModel" />
<Style TargetType="TextBlock" x:Key="NameStyle">
<Setter Property="FontSize" Value="20"/>
<Setter Property="FontWeight" Value="Bold"/>
<Setter Property="FontFamily" Value="Consolas"/>
<Setter Property="Foreground" Value="DarkMagenta"/>
</Style>
<Style TargetType="TextBlock" x:Key="MessageStyle">
<Setter Property="FontSize" Value="16"></Setter>
<Setter Property="FontFamily" Value="Segoe UI"/>
<Setter Property="TextWrapping" Value="Wrap"/>
</Style>
</UserControl.Resources>
<Grid Width="{Binding (UserControl.Width), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}">
<ListBox ItemsSource="{Binding Exceptions}" d:DataContext="{Binding Source={StaticResource DesignViewModel}}" Width="{Binding (Grid.Width), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Grid}}}">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Width="{Binding (ListBox.Width), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBox}}}">
<TextBlock Text="{Binding Name}" Style="{StaticResource NameStyle}" />
<TextBlock Text="{Binding Message}" Style="{StaticResource MessageStyle}" />
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
</UserControl>

It sounds like you should get rid of all the width bindings and just put a margin around your StackPanel, e.g:
<StackPanel Margin="10" >
--- UPDATED --
Frustrating is the right word. Okay, here's the solution.
(1) Replace StackPanel with Grid.
(2) In the ListBox, set ScrollViewer.HorizontalScrollBarVisibility to "Disabled"
<ListBox ScrollViewer.HorizontalScrollBarVisibility="Disabled">
<Grid Margin="10" >
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock Grid.Row="0" Text="Title" Style="{StaticResource NameStyle}" />
<TextBlock Grid.Row="1" TextWrapping="Wrap" Text="lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text lots of text " Style="{StaticResource MessageStyle}" />
</Grid>
</ListBox>
This should do the trick.

I finally got it! I found a similar question and adopted its answer to my needs:
<Grid >
<ListBox ItemsSource="{Binding Exceptions}" d:DataContext="{Binding Source={StaticResource DesignViewModel}}" Margin="30" >
<ListBox.ItemContainerStyle>
<Style TargetType="ListBoxItem">
<Setter Property="Width" Value="{Binding (Grid.ActualWidth), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Grid}}}" />
</Style>
</ListBox.ItemContainerStyle>
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel>
<TextBlock Text="{Binding Name}" Style="{StaticResource NameStyle}" />
<TextBlock Text="{Binding Message}" Style="{StaticResource MessageStyle}" />
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
The trick was to bind the Width of the ListBoxItem to the parent. Holy cow! This was tricky. Now it also nicely scales when resizing the window.

Instead of referring to ListBox, refer width of ListBoxItem
<StackPanel Width="{Binding (ListBoxItem.Width), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}}}">

The reason for the overlap is due to the default template for a ListBoxItem, which has Padding set to "4,1" and a BorderThickness of "1" on both itself and the inner Border object. You can use a tool such as WPF Snoop to see this.
The solution is to bind to the ActualWidth of the next level of container, in this case the ContentPresenter.
<ListBox ItemsSource="{Binding DataItems}"
ScrollViewer.HorizontalScrollBarVisibility="Disabled"
HorizontalContentAlignment="Stretch">
<ListBox.ItemTemplate>
<DataTemplate>
<Grid Background="Yellow"
Width="{Binding RelativeSource={RelativeSource FindAncestor,
AncestorType={x:Type ContentPresenter}},
Path=ActualWidth}">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<TextBlock Grid.Row="0" Text="{Binding Id}" />
<TextBlock Grid.Row="1" Text="{Binding Description}" TextWrapping="Wrap" />
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
Note the properties set on the ListBox, which prevent the content growing beyond the width of the ListBox, and forcing the content to resize as the ListBox width is increased.

XAML GridRow doesn't scale automatically when ErrorTemplate is displayed

I have a XAML window with a TextBox, and this TextBox has an ErrorTemplate.
The ErrorTemplate is shown below, and as you can see, I have an AdornedElementPlaceholder, followed by a textbox whose Text field is bound to the ErrorContent:
<ControlTemplate x:Key="ValidationErrorTemplateTextBlock" TargetType="{x:Type Control}">
<Border BorderBrush="Red" BorderThickness="1">
<StackPanel Orientation="Vertical">
<AdornedElementPlaceholder Name="AdornedElementPlaceholder" />
<TextBlock Text="{Binding ElementName=AdornedElementPlaceholder, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}"
FontSize="10"
Background="Red"
Foreground="White"
Padding="2" />
</StackPanel>
</Border>
</ControlTemplate>
<TextBox IsEnabled="{Binding SendMessage}"
Text="{Binding AutoMessageSubject, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}"
Style="{StaticResource StyleBase}"
Validation.ErrorTemplate="{StaticResource ValidationErrorTemplateTextBlock}"
HorizontalAlignment="Stretch"
Grid.Row="3"
Grid.Column="1"
Grid.ColumnSpan="2" />
This works fine, except for one thing: the TextBox is inside a GridRow, with a Height="Auto". The row scales itself based on the textbox, but when the ErrorTemplate appears, with an extra TextBox on the bottom - the GridRow doesn't scale up to contain the new TextBox, and the new TextBox overlaps the elements below it.
How can I solve this?

Validation.ErrorTemplate: Gets or sets the ControlTemplate used to generate validation error feedback on the adorner layer.
This means that if you use Validation.ErrorTemplate, the validation errors are displayed on the layer above usual content, so the "second" TextBlock is displayed over the grid, not within the grid cell.
I would implement INotifyDataErrorInfo instead of semi-obsolete IDataErrorInfo, use a custom textbox style, and bind the visibility of the second TextBlock to HasErrors property. The example below uses a ToolTip instead of the second TextBlock:
<Style TargetType="{x:Type TextBox}">
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="True">
<Setter Property="ToolTip">
<Setter.Value>
<ToolTip DataContext="{Binding RelativeSource={RelativeSource Self}, Path=PlacementTarget}">
<ItemsControl DisplayMemberPath="ErrorContent" ItemsSource="{Binding Path=(Validation.Errors)}" />
</ToolTip>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>

You might want to try adding Row/Column Definitions:
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>

Control template: how to create bindings

So, I have a datagrid that has different colour cells depending on the cell's value.
I also have a tooltip that displays some further information. This all works fine.
I, however, would like to alter the tooltip to show further information and also to be the same colour as the cell. So, I thought it would be wise to create a custom style for my tool tips. So, I have the below code.
<Style TargetType="ToolTip">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToolTip">
<Border CornerRadius="15,15,15,15"
BorderThickness="3,3,3,3"
Background="#AA000000"
BorderBrush="#99FFFFFF"
RenderTransformOrigin="0.5,0.5">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="2*"/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<TextBlock Grid.Row="0"/>
<TextBlock Grid.Row="1"/>
<TextBlock Grid.Row="2"/>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
I have an object shown below that is bound to my datagrid. I want to bind the three properties to the three textboxes in my tooltip.
class MyTask
{
public string Name;
public int Code;
public string Description;
}
In my DataGrid I do the following to bind my data to my datagrid
ItemsSource="{Binding TaskList}"
Then in the DataGridTextColumn I bind to a property like below
DataGridTextColumn Header="Code" Binding="{Binding Code}"
This makes sense to me. I am however at a loss to see how I use binding when creating my custom tooltip. I read that I can use templatebinding. I still don't understand how my tooltip will bind to my object of type MyTask in my xaml above?
Update - hopefully make my question clearer
I want to know how to create the bindings in my control template (for the 3 textboxes) and then in the main part of my code how I bind to these text boxes. I then would like to know how to create a binding for the background colour of my control template, I believe this is something to do with relativesource?
When I'm reading other examples (changing the Template Property) I see lines like below. I don't really understand why you have to do it? Is it a case of if you didn't right the line below you wouldn't be able to create a binding on the Padding property?
<Border Padding="{Binding Padding}" ...>

You don't need TemplateBindng, as that is used for setting up the resulting template object to layout based on dynamically using the properties of the implementing control. See this CodePlex article for a good example of when you'd need such functionality.
You simply need to set the bindings of your TextBlock elements within your ToolTip. You don't really need a Template at all in this case, except that since you are using the same ToolTip across all your column cells, it will help you out as you don't need to copy-paste the same code three times. You are after something similar to this article, Tooltip in DataGrid in WPF.
A solution which would work specifically to your case would be like:
<DataGrid Name="TestGrid1" AutoGenerateColumns="False">
<DataGrid.Columns>
<DataGridTemplateColumn Header="Name">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBlock Text="{Binding Name}">
<TextBlock.ToolTip>
<ToolTip />
</TextBlock.ToolTip>
</TextBlock>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
<DataGridTemplateColumn Header="Code">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBlock Text="{Binding Code}">
<TextBlock.ToolTip>
<ToolTip />
</TextBlock.ToolTip>
</TextBlock>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
<DataGridTemplateColumn Header="Description">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBlock Text="{Binding Description}">
<TextBlock.ToolTip>
<ToolTip />
</TextBlock.ToolTip>
</TextBlock>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
</DataGrid.Columns>
<DataGrid.Resources>
<Style TargetType="ToolTip">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToolTip">
<Border CornerRadius="15,15,15,15"
BorderThickness="3,3,3,3"
Background="#AA000000"
BorderBrush="#99FFFFFF"
RenderTransformOrigin="0.5,0.5">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="2*"/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<TextBlock Grid.Row="0" Text="{Binding Name}"/>
<TextBlock Grid.Row="1" Text="{Binding Code}"/>
<TextBlock Grid.Row="2" Text="{Binding Description}"/>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</DataGrid.Resources>
</DataGrid>
You set the ToolTip property within the CellTemplate so that the resulting ToolTip that pops up has the same DataContext as the active row in the DataGrid. This way, you can simply do your property bindings as normal within your ToolTip ContentTemplate, since it has access to all the same properties as does your DataGrid for the row.

To use underlying DataGridCell Background as ToolTip Background, bind your Border Background as Background="{Binding PlacementTarget.Background, RelativeSource={RelativeSource AncestorType=ToolTip, Mode=FindAncestor}}" .
You are trying to show all the fields in the tooltip of a cell. That doesn't make sense. But still you can do that easily using
PlacementTarget property, which gives you the underlying Visual element. ContextMenu, Popup too expose this property.
PlacementTarget.DataContext will give you the underlying MyTask object.
PlacementTarget.Content will give you the content of the corresponding DataGridCell, in your case it will be TextBlock.
So, if you want to show 3 fields in your cell's tooltip, below code will work for you using point number 2 above.
<DataGrid.CellStyle>
<Style TargetType="DataGridCell">
<Setter Property="Background" Value="Tomato"/>
<Setter Property="ToolTip">
<Setter.Value>
<ToolTip>
<ToolTip.Style>
<Style TargetType="ToolTip">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToolTip">
<Border CornerRadius="15,15,15,15"
BorderThickness="3,3,3,3"
Background="{Binding PlacementTarget.Background, RelativeSource={RelativeSource AncestorType=ToolTip, Mode=FindAncestor}}"
BorderBrush="#99FFFFFF"
RenderTransformOrigin="0.5,0.5">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="2*"/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<TextBlock Grid.Row="0" Text="{Binding PlacementTarget.DataContext.Name, RelativeSource={RelativeSource AncestorType=ToolTip, Mode=FindAncestor}}"
/>
<TextBlock Grid.Row="0" Text="{Binding PlacementTarget.DataContext.Code, RelativeSource={RelativeSource AncestorType=ToolTip, Mode=FindAncestor}}"
/>
<TextBlock Grid.Row="0" Text="{Binding PlacementTarget.DataContext.Description, RelativeSource={RelativeSource AncestorType=ToolTip, Mode=FindAncestor}}"
/>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ToolTip.Style>
</ToolTip>
</Setter.Value>
</Setter>
</Style>
</DataGrid.CellStyle>
And, if you want to show only corresponding cell's field in your cell's tooltip, then remove the remaining 2 textblocks, and use only one as :
<TextBlock Text="{Binding PlacementTarget.Content.Text, RelativeSource={RelativeSource AncestorType=ToolTip, Mode=FindAncestor}}" />
If you want to show all 3 fields, then apply the ToolTip to DataGridRow using DataGrid.RowStyle. No change in code would be needed.

What about using a color property in MyTask?
class MyTask
{
public string Name { get; set; }
public int Code { get; set; }
public string Description { get; set; }
public SolidColorBrush Color { get; set; }
}
And binding to the color property:
<Style TargetType="ToolTip">
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="HasDropShadow" Value="True"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToolTip">
<Border Name="Border" Background="{Binding Color}" BorderBrush="{StaticResource SolidBorderBrush}" BorderThickness="1" Width="{TemplateBinding Width}" Height="{TemplateBinding Height}">
<StackPanel>
<TextBlock Text="{Binding Name}" />
<TextBlock Text="{Binding Code}" />
<TextBlock Text="{Binding Description}" />
</StackPanel>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Why don't you go for DataTemplate of tool tip like give it's a key and apply it in your cell tool tip style.
<Style TargetType="ToolTip" x:Key="ToolTipStyle">
<Setter Property="ContentTemplate">
<Setter.Value>
<DataTemplate TargetType="ToolTip">
<Border CornerRadius="15,15,15,15"
BorderThickness="3,3,3,3"
Background="#AA000000"
BorderBrush="#99FFFFFF"
RenderTransformOrigin="0.5,0.5">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="2*"/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<TextBlock Grid.Row="0"/>
<TextBlock Grid.Row="1"/>
<TextBlock Grid.Row="2"/>
</Grid>
</Border>
</DataTemplate>
</Setter.Value>
</Setter>
</Style>
Now, you can bind your property to textblock too.