Converter With Multiple Parameters

While the above answers may be feasible, they seem to be overly complicated. Simply use an IMultiValueConverter with an appropriate MultiBinding in the XAML code. Assuming that your ViewModel has the properties FirstValue, SecondValue, and ThirdValue, which are an int, a double, and a string, respectively, a valid multi converter might look like this:

C#

public class MyMultiValueConverter : IMultiValueConverter {
  public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
    int firstValue = (int)values[0];
    double secondValue = (double)values[1];
    string thirdValue = (string)values[2];

    return "You said " + thirdValue + ", but it's rather " + firstValue * secondValue;
  }

  public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture) {
    throw new NotImplementedException("Going back to what you had isn't supported.");
  }
}

XAML

<TextBlock.Text>
  <MultiBinding Converter="{StaticResource myNs:MyMultiValueConverter}">
    <Binding Path="FirstValue" />
    <Binding Path="SecondValue" />
    <Binding Path="ThirdValue" />
  </MultiBinding>
</TextBlock.Text>

Since it requires neither fumbling with the ProvideValue method required by MarkupExtension, nor the specification of a DependencyObject inside(!) a converter, I do believe that this is the most elegant solution.

Converters always implement IValueConverter. That means a call to Convert or ConvertBack passes a single additional parameter. That parameter is extracted from the XAML.

As Hitesh Patel suggests there is nothing to stop you putting more than one value into the parameter, so long as you have a delimiter to separate them out later, but you cannot use a comma as that delimits the XAML!

e.g.

XAML

<TextBlock Text="{Binding Path=ReleaseDate, Mode=OneWay,
                        Converter={StaticResource MyConverter}, 
                        ConverterParameter=Param1|Param2}" />

Converter

public object Convert(object value, Type targetType, object parameter,
    System.Globalization.CultureInfo culture)
{
    string parameterString = parameter as string;
    if (!string.IsNullOrEmpty(parameterString))
    {
        string[] parameters = parameterString.Split(new char[]{'|'});
        // Now do something with the parameters
    }
}

Note, I have not checked it to see if a Pipe "|" character is valid in XAML there (should be), but if not just choose another character that does not clash.

Later versions of .Net do not require a character array for the simplest version of Split, so you can use this instead:

string[] parameters = parameterString.Split('|');

Addendum:

A trick eBay used to use in urls, years ago, was to delimit data in the URL with QQ. A double-Q does not naturally occur in text data. If you ever get stuck for a text delimiter that will avoid encoding issues just use QQ... This will not work with split though (which requires single characters, but nice to know) :)