Selecting a value in combobox

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C#

WPF

I am populating a combobox in WPF like this

_cviewPreferencesPresenter.ListOfOVUnits();

cmbOVUnits.SelectedItem = _cstrSelectdOVUnit;
            _cstrDisplayedOVUnitID = cmbOVUnits.SelectedValue.ToString();

After the combobox loading i want to see the default value selected but i am not getting any value cmbOVUnits.SelectedItem is null.But the combobox is getting populated.

When i select some value in the combobox and click on apply button the value is not pertaining it is going to first value.

public void PopulateOVList(List<KeyValuePair<string, string>> lstOVData)
    {
        try
        {
            cmbOVUnits.DataContext = lstOVData;
            cmbOVUnits.DisplayMemberPath = "Value";
            cmbOVUnits.SelectedValuePath = "Key";
        }
        catch (FlukeException)
        {
            throw;
        }
        catch (Exception excException)
        {
            if (FlukeConstants.IsLoggerEnabled)
            {
                Common.LogApplicationExceptionMsg(excException);
            }
            throw new FlukeException(SeverityLevel.Fatal, LogLevel.Debug, excException,
                    FlukeExceptionConstants.FLK_ERR_COMMON_APPLICATIONEXCEPTION);
        }
    }

here i can see cmbOVUnits cmbOVUnits = {System.Windows.Controls.ComboBox Items.Count:0} is '0', even i though i am getting values in lstOVData.The DataContext have the values like cmbOVUnits.DataContext = Count = 14.

So obviously cstrDisplayedOVUnitID value will be null.How can i solve this?