i'he 3 combobox for country,state and cities.when i select one country that coressponing states will display in 2nd combobox,and when i select one state that coressponing cities will display in...

The link given in solution 1 is good but it is for HTML with JavaScript.
For C# the following procedure can be used for Windows Forms
Create DataRelations between the Country, State and City DataTables using either the DataSet designer or programmatically.

Relation : CountryState
Parent Table: Country and primary key: CountryID
Child Table: State and Foreign key: CountryID
Relation : StateCity
Parent Table: State and primary key: StateID
Child Table: City and Foreign key: StateID

//Create a BindingSource for Country ComboBox using Country table from DataSet1
BindingSource CountryBindingSource = New BindingSource(DataSet1, "Country");
//Now create a BindingSource with CountryBindingSource as the parent using the relation 
BindingSource StateBindingSource = New BindingSource(CountryBindingSource, "CountryState");
//Then create a BindingSource with StateBindingSource as the parent using the relation 
BindingSource CityBindingSource = New BindingSource(StateBindingSource, "StateCity");

//Set the DataSource, DisplayMember and ValueMember properties of the ComboBoxes as below
comboBox1.DataSource = CountryBindingSource;
comboBox1.DisplayMember = "Country";
comboBox1.ValueMember = "CountryID";
comboBox2.DataSource = StateBindingSource;
comboBox2.DisplayMember = "State";
comboBox2.ValueMember = "StateID";
comboBox3.DataSource = CityBindingSource;
comboBox3.DisplayMember = "City";
comboBox3.ValueMember = "CityID";