Conditional binding from view model to view in WPF using MVVM pattern

Question

I am developing a WPF application using MVVM pattern. I have a combo in the view and two lists in the viewmodel (projects and organizations). Depending on the organizations list items I have to bind the the name of the organization or not. For example if the Count property of the organizations list is 1 the combobox item have to be "ProjectName" , and if the Count property of the organizations list is greater than 1 the combobox item should look like "ProjectName - OrganizationName" . This is the XAML code I have:

<ComboBox x:Name="textBox3" Grid.Row="2" Grid.Column="1" Grid.ColumnSpan="2" ItemsSource="{Binding Path=Projects}" DisplayMemberPath="Name" IsSynchronizedWithCurrentItem="True" SelectedItem="{Binding Path=SelectedProject}">
        </ComboBox>

How should I achieve this purpose. I hope for a little help. Cheers.

I added the property projectFullName in the viewmodel but I got an empty combobox:

 public string ProjectFullName
    {
        get
        {
            if (this.organizations.ToList().Count > 1)
            {
                this.projectFullName = string.Format("{}{0} - {1}", this.selectedProject.Name, this.organizations.First(org => org.Id == this.selectedProject.OrganizationId).Name);
            }
            else if (this.organizations.ToList().Count == 1)
            {
                this.projectFullName = this.selectedProject.Name;
            }
            return this.projectFullName;
        }
    }

XAML code:

<ComboBox x:Name="textBox3" Grid.Row="2" Grid.Column="1" Grid.ColumnSpan="2" ItemsSource="{Binding Path=Projects}" DisplayMemberPath="{Binding Path=ProjectFullName}" IsSynchronizedWithCurrentItem="True" SelectedItem="{Binding Path=SelectedProject}">

        </ComboBox>

Answer 1

you have several options to implement this, but in my opinion the best is:

Add a property to your Data Context, the will be called "FullName" or something. That will return: (Pseudo) if Projects count > 0 then return Name + '-' + ProjectName else return Name

then bind DisplayMemberPath to FullName.

Answer 2

Datatrigger is indeed your friend. Make sure the ComboBox does not set the DisplayMemberPath, because that will override the style setters.

<Style x:Key="MyStyle"  TargetType="ComboBox">
        <Setter Property="DisplayMemberPath" Value="DefaultName"/>
        <Style.Triggers>
            <DataTrigger Binding="{Binding Items.Count}" Value="1">
                <Setter Property="DisplayMemberPath" Value="OtherName"/>
            </DataTrigger>
        </Style.Triggers>
 </Style>