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How to use indexed properties in COM interop programming

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Indexed properties work together with other features in C#, such as named and optional arguments, a new type (dynamic), and embedded type information, to enhance Microsoft Office programming.

Important

VSTO (Visual Studio Tools for Office) relies on the .NET Framework. COM add-ins can also be written with the .NET Framework. Office Add-ins cannot be created with .NET Core and .NET 5+, the latest versions of .NET. This is because .NET Core/.NET 5+ cannot work together with .NET Framework in the same process and may lead to add-in load failures. You can continue to use .NET Framework to write VSTO and COM add-ins for Office. Microsoft will not be updating VSTO or the COM add-in platform to use .NET Core or .NET 5+. You can take advantage of .NET Core and .NET 5+, including ASP.NET Core, to create the server side of Office Web Add-ins.

In earlier versions of C#, methods are accessible as properties only if the get method has no parameters and the set method has one and only one value parameter. However, not all COM properties meet those restrictions. For example, the Excel Range[] property has a get accessor that requires a parameter for the name of the range. In the past, because you couldn't access the Range property directly, you had to use the get_Range method instead, as shown in the following example.

// Visual C# 2008 and earlier.
var excelApp = new Excel.Application();
// . . .
Excel.Range targetRange = excelApp.get_Range("A1", Type.Missing);

Indexed properties enable you to write the following instead:

// Visual C# 2010.
var excelApp = new Excel.Application();
// . . .
Excel.Range targetRange = excelApp.Range["A1"];

The previous example also uses the optional arguments feature, which enables you to omit Type.Missing.

Indexed properties enable you to write the following code.

// Visual C# 2010.
targetRange.Value = "Name";

You can't create indexed properties of your own. The feature only supports consumption of existing indexed properties.

Example

The following code shows a complete example. For more information about how to set up a project that accesses the Office API, see How to access Office interop objects by using C# features.

// You must add a reference to Microsoft.Office.Interop.Excel to run
// this example.
using System;
using Excel = Microsoft.Office.Interop.Excel;

namespace IndexedProperties
{
    class Program
    {
        static void Main(string[] args)
        {
            CSharp2010();
        }

        static void CSharp2010()
        {
            var excelApp = new Excel.Application();
            excelApp.Workbooks.Add();
            excelApp.Visible = true;

            Excel.Range targetRange = excelApp.Range["A1"];
            targetRange.Value = "Name";
        }

        static void CSharp2008()
        {
            var excelApp = new Excel.Application();
            excelApp.Workbooks.Add(Type.Missing);
            excelApp.Visible = true;

            Excel.Range targetRange = excelApp.get_Range("A1", Type.Missing);
            targetRange.set_Value(Type.Missing, "Name");
            // Or
            //targetRange.Value2 = "Name";
        }
    }
}

See also