The ICustomFormatter interface includes a single method, ICustomFormatter.Format(string, object, IFormatProvider). When this interface is implemented by a reference or value type, the ICustomFormatter.Format(string, object, IFormatProvider) method returns a custom-formatted string representation of an object's value.

Typically, the ICustomFormatter interface is implemented with the IFormatProvider interface to customize the behavior of two .NET Framework composite string formatting methods that include an IFormatProvider parameter. Specifically, the ICustomFormatter interface can provide custom formatting of the value of an object passed to the string.Format(IFormatProvider, string, Object[]) and System.Text.StringBuilder.AppendFormat(IFormatProvider, string, Object[]) methods.

Providing a custom representation of an object's value requires that you do the following:

The .NET Framework method will then use your custom formatting instead of its own.