I have three Combo boxes.I am fetching value of each combobox from the database and each combobox is dependent on the other. Currently, I am populating the combobox on the SelectionChanged() event. I am populating first combobox in the constructor of MainWindow.
So when I select a value from first combobox, the selectionchanged event is called. then second combox is poplulated. When I select a value from second combobox, third combobox is populated on selectionChanged event. After selecting value from third combo box, I am populating a data grid.
When I try and change value of first combobox after selecting value from third combobox, I get an error.
Object reference not set to an instance of an object.!
I am trying to bind the comboboxes to each other, but I cannot figure out a way to do that being new to wpf.
I have attached code of combobox for reference.
public MainWindow()
{
InitializeComponent();
Fill_Combo();
}
//adding values in first combo box
void Fill_Combo()
{
try
{
string query = "select column_name from table_name";
if (conn.State == ConnectionState.Closed)
{
conn.Open();
}
OracleCommand command = new OracleCommand();
command.Connection = conn;
command.CommandText = query;
OracleDataReader reader = command.ExecuteReader();
while (reader.Read())
{
String str = reader.GetString(0);
//source_name is name of combobox1
source_name.Items.Add(str);
}
reader.Dispose();
}
catch (Exception e)
{
MessageBox.Show(e.StackTrace + " :: in fill_combo");
}
}
// adding value in second combo box based on value in first combobox
private void source_name_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
string query = "select column from table where source_name='" + source_name.SelectedItem.ToString() + "'";
try
{
OracleConnection connection = new OracleConnection(connectionString);
connection.Open();
OracleCommand command = new OracleCommand();
command.Connection = connection;
command.CommandText = query;
OracleDataReader reader = command.ExecuteReader();
//clearing previous values in combobox2
//target_box is the name of second combo box
target_box.Items.Clear();
while (reader.Read())
{
String str = reader.GetString(0);
//adding values in second combobox
target_box.Items.Add(str);
}
reader.Dispose();
connection.Close();
}
catch (Exception exp)
{
MessageBox.Show(exp.Message + "source_name namespace SelectionChangedEventArgs chanegs");
}
}
private void target_box_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
try
{
if (target_box.SelectedItem.ToString() != null && target_box.SelectedItem.ToString() != "Target system")
{
string query = "select columnname from map_pair_attributes where source_name='" + source_name.SelectedItem.ToString() + "'"
+ "and target_name = '" + target_box.SelectedItem.ToString() + "'";
OracleConnection connection = new OracleConnection(connectionString);
connection.Open();
OracleCommand command = new OracleCommand();
command.Connection = connection;
command.CommandText = query;
OracleDataReader reader = command.ExecuteReader();
attribute_box.Items.Clear();
while (reader.Read())
{
String str = reader.GetString(0);
//attribute box is name of third combobox
attribute_box.Items.Add(str);
}
reader.Dispose();
connection.Close();
connection.Dispose();
}
}
catch (Exception ex)
{
MessageBox.Show("in target box selection changed!" + ex.StackTrace + ex.Message);
}
}
//atributebox is name of third combobox
private void attribute_box_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
try
{
String query = "select column_name from table where source_name='" + source_name.SelectedItem.ToString() + "' and target_name ='" +
target_box.SelectedItem.ToString() + "' and attribute_name = '" + attribute_box.SelectedItem.ToString() + "'";
OracleConnection conn = new OracleConnection(connectionString);
conn.Open();
DataSet ds = new DataSet();
OracleDataAdapter da = new OracleDataAdapter(query, conn);
da.Fill(ds, "table_name");
grid1.ItemsSource = ds.Tables["table_name"].DefaultView;
conn.Close();
}
catch (Exception ex)
{
MessageBox.Show("attribute box selection changed!" + ex.Message);
}
}
.XAML file
<ComboBox x:Name="target_box" HorizontalAlignment="Left" Margin="179,49,0,0" VerticalAlignment="Top" Width="120"
IsEditable="true" IsReadOnly="True" Text="Target system"
Focusable="True" SelectionChanged="target_box_SelectionChanged" />
<ComboBox x:Name="source_name" HorizontalAlignment="Left" Margin="19,49,0,0" VerticalAlignment="Top" Width="120"
IsEditable="true" IsReadOnly="True" Text="Source system" Focusable="True" SelectionChanged="source_name_SelectionChanged"/>
<ComboBox x:Name="attribute_box" HorizontalAlignment="Left" Margin="363,49,0,0" VerticalAlignment="Top" Width="120"
IsEditable="true" IsReadOnly="True" Text="Attribute" Focusable="True" SelectionChanged="attribute_box_SelectionChanged" />
Can someone please help me with this. I think binding can solve the problem, but I do not know how to bind the comboboxes.
Thanks!
答案 0 :(得分：1)
When you select a new item in the lines = defaultdict(dict)
with open("file_parse.txt") as f:
for line in f:
parts = line.split()
key = tuple(parts[1:3]) # unique key with servername and datetime
lines[key][parts[0]] = parts[3]
lines[key]["servername"] = parts[1]
lines[key]["datetime"] = parts[3]
res = list(lines.values())
print(res)
# [
# {'cpudile': 'NaN', 'cpunice': '0.0', 'cpunumber': '40.0', 'cpusystem': '0.0', 'cpuuser': '0.0', 'cpuwait': '0.0', 'datetime': 'NaN', 'servername': '1438084445'},
# {'cpudile': 'NaN', 'cpunice': '0.0', 'cpunumber': '40.0', 'cpusystem': '0.0', 'cpuuser': '0.0', 'cpuwait': '0.0', 'datetime': 'NaN', 'servername': '1438084440'},
# {'cpudile': 'NaN', 'cpunice': '0.0', 'cpunumber': '40.0', 'cpusystem': '0.0', 'cpuuser': '0.0', 'cpuwait': '0.0', 'datetime': 'NaN', 'servername': '1438084450'}
# ]
, the ComboBox is first set to null.
Try adding in checks for ComboBox.SelectedItem = null in each event handler before trying you are calling ToString() on the object.
e.g.
ComboBox.SelectedItemThen replace your ToString null checks
if(source_name.SelectedItem == null) return;
with
if (target_box.SelectedItem.ToString() != null