How to access datagrid template column textbox text WPF C#

To find a control in a DataGrid template column, you should use FindChild():

    public static T FindChild<T>(DependencyObject parent, string childName) where T : DependencyObject
    {
        if (parent == null)
        {
            return null;
        }

        T foundChild = null;

        int childrenCount = VisualTreeHelper.GetChildrenCount(parent);

        for (int i = 0; i < childrenCount; i++)
        {
            var child = VisualTreeHelper.GetChild(parent, i);
            T childType = child as T;

            if (childType == null)
            {
                foundChild = FindChild<T>(child, childName);

                if (foundChild != null) break;
            }
            else
                if (!string.IsNullOrEmpty(childName))
                {
                    var frameworkElement = child as FrameworkElement;

                    if (frameworkElement != null && frameworkElement.Name == childName)
                    {
                        foundChild = (T)child;
                        break;
                    }
                    else
                    {
                        foundChild = FindChild<T>(child, childName);

                        if (foundChild != null)
                        {
                            break;
                        }
                    }
                }
                else
                {
                    foundChild = (T)child;
                    break;
                }
        }

        return foundChild;
    }

For example I have this template column in MyDataGrid:

<DataGridTemplateColumn Width="1.5*" IsReadOnly="False">
    <DataGridTemplateColumn.Header>
        <TextBlock Text="Sample" ToolTip="{Binding Path=Text, RelativeSource={RelativeSource Self}}" FontSize="14" />
     </DataGridTemplateColumn.Header>

     <DataGridTemplateColumn.CellTemplate>
         <DataTemplate>
             <TextBlock x:Name="MyTextBlock" Text="Hello!" />
         </DataTemplate>
    </DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>

Access it from the code, you can:

TextBlock MyTextBlock = FindChild<TextBlock>(MyDataGrid, "MyTextBlock");

MessageBox.Show(MyTextBlock.Text);

Note: Always use FindChild only when the control will be fully loaded, otherwise it will not find it and give null. In this case, I put this code in the event ContentRendered (Window) which says that all the contents of the window successfully load (even the event MyDataGrid_Loaded not have access to MyTextBlock, because it is not yet loaded):

    private void Window_ContentRendered(object sender, EventArgs e)
    {
        TextBlock MyTextBlock = FindChild<TextBlock>(MyDataGrid, "MyTextBlock");

        MessageBox.Show(MyTextBlock.Text);
    }

EDIT1:

To access the control of selected row to add event SelectionChanged to DataGrid in which to function, which will give a selected row:

    private void MyDataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        try
        {
            var row_list = GetDataGridRows(MyDataGrid);

            foreach (DataGridRow single_row in row_list)
            {
                if (single_row.IsSelected == true)
                {
                    TextBlock MyTextBlock = FindChild<TextBlock>(single_row, "MyTextBlock");

                    MessageBox.Show(MyTextBlock.Text);
                }
            }
        }

        catch 
        {
            throw new Exception("Can't get access to DataGridRow");
        }
    }

Listing of GetDataGridRows():

    public IEnumerable<DataGridRow> GetDataGridRows(DataGrid grid)
    {
        var itemsSource = grid.ItemsSource as IEnumerable;

        if (null == itemsSource)
        {
            yield return null; 
        }

        foreach (var item in itemsSource)
        {
            var row = grid.ItemContainerGenerator.ContainerFromItem(item) as DataGridRow;

            if (null != row)
            {
                yield return row; 
            }
        }
    }

EDIT2:

To get ALL the items I rewrote the function FindChild():

    public static void FindChildGroup<T>(DependencyObject parent, string childName, ref List<T> list) where T : DependencyObject
    {
        // Checks should be made, but preferably one time before calling.
        // And here it is assumed that the programmer has taken into
        // account all of these conditions and checks are not needed.
        //if ((parent == null) || (childName == null) || (<Type T is not inheritable from FrameworkElement>))
        //{
        //    return;
        //}

        int childrenCount = VisualTreeHelper.GetChildrenCount(parent);

        for (int i = 0; i < childrenCount; i++)
        {
            // Get the child
            var child = VisualTreeHelper.GetChild(parent, i);

            // Compare on conformity the type
            T child_Test = child as T;

            // Not compare - go next
            if (child_Test == null)
            {
                // Go the deep
                FindChildGroup<T>(child, childName, ref list);
            }
            else
            {
                // If match, then check the name of the item
                FrameworkElement child_Element = child_Test as FrameworkElement;

                if (child_Element.Name == childName)
                {
                    // Found
                    list.Add(child_Test);
                }

                // We are looking for further, perhaps there are
                // children with the same name
                FindChildGroup<T>(child, childName, ref list);
            }
        }

        return;
    }

Calling this new function:

   private void Window_ContentRendered(object sender, EventArgs e)
   {
        // Create the List
        List<TextBlock> list = new List<TextBlock>();

        // Find all elements
        FindChildGroup<TextBlock>(MyDataGrid, "MyTextBlock", ref list);
        string text = "";

        // Print
        foreach (TextBlock elem in list)
        {
            text += elem.Text + "\n";
        }

        MessageBox.Show(text, "Text in TextBlock");
   }

Generally, this practice is not the best … to get the items (such as all or selected), you can contact directly to the list which stores your data (like ObservableCollection). Also, it is useful events such as PropertyChanged.