MAPI property IDs 

Outlook .MSG files contain three kinds of MAPI properties:

• Tagged properties - identified by a tag (MsgPropertyTag enum).
• Numerical named properties - with numerical identifier (MsgPropertyId and MsgPropertySet enums).
• String named properties - with string identifier (string and MsgPropertySet enum).

// load an MsgMessage
// ...

// display all MAPI properties of the MSG message
foreach (MsgProperty prop in message.Properties)
{
    switch (prop.Kind)
    {
        case MsgPropertyKind.Tagged:
            Console.WriteLine("Tagged: {0}", prop.Tag);
            break;
        case MsgPropertyKind.NumericalNamed:
            Console.WriteLine("Numerical: {0}", prop.Id);
            break;
        case MsgPropertyKind.StringNamed:
            Console.WriteLine("String: {0}", prop.Name);
            break;
    }
}

// get a 'Tagged' property
MsgProperty subject = message.Properties[MsgPropertyTag.Subject];

// get a 'Numerical-named' property
MsgProperty account = message.Properties[MsgPropertyId.InternetAccountName];

// get a 'String-named' property
MsgProperty contentType = message.Properties["content-type"];

' load an MsgMessage
' ...

' display all MAPI properties of the MSG message
For Each prop As MsgProperty In message.Properties

    Select Case prop.Kind
        Case MsgPropertyKind.Tagged
            Console.WriteLine("Tagged: {0}", prop.Tag)
        Case MsgPropertyKind.NumericalNamed
            Console.WriteLine("Numerical: {0}", prop.Id)
        Case MsgPropertyKind.StringNamed
            Console.WriteLine("String: {0}", prop.Name)
    End Select
Next

' get a 'Tagged' property
Dim subject As MsgProperty = message.Properties(MsgPropertyTag.Subject)

' get a 'Numerical-named' property
Dim account As MsgProperty = message.Properties(MsgPropertyId.InternetAccountName)

' get a 'String-named' property
Dim contentType As MsgProperty = message.Properties("content-type")

MAPI property data types 

MAPI properties are of various data types defined by MsgPropertyDataType enum (which corresponds to [MS-OXCDATA] specification). The particular data type of a MAPI property can be determined using MsgProperty.DataType property.

MsgProperty.Value returns and accepts an instance of object, but it can be safely casted to a strongly-typed value based on MsgProperty.DataType property.

When setting MsgProperty.Value the type of the value must correspond to the type specified by MsgProperty.DataType property.

// load an MsgMessage
// ...

// get the 'Subject' property
MsgProperty subjectProp = message.Properties[MsgPropertyTag.Subject];
if (subjectProp == null)
    throw new InvalidDataException("Subject not found.");

// get the property value based on its data type
string subject;
switch (subjectProp.DataType)
{
    case MsgPropertyDataType.String:
    case MsgPropertyDataType.String8:
        subject = (string)subjectProp.Value;
        break;
    case MsgPropertyDataType.Binary:
        subject = Encoding.UTF8.GetString((byte[])subjectProp.Value);
        break;
    default:
        throw new InvalidDataException("Subject data format is unknown.");
}

' load an MsgMessage
' ...

' get the 'Subject' property
Dim subjectProp As MsgProperty = message.Properties(MsgPropertyTag.Subject)
If subjectProp Is Nothing Then
    Throw New InvalidDataException("Subject not found.")
End If

' get the property value based on its data type
Dim subject As String

Select Case subjectProp.DataType
    Case MsgPropertyDataType.String, MsgPropertyDataType.String8
        subject = CStr(subjectProp.Value)
    Case MsgPropertyDataType.Binary
        subject = Encoding.UTF8.GetString(CType(subjectProp.Value, Byte()))
    Case Else
        Throw New InvalidDataException("Subject data format is unknown.")
End Select

Getting and setting values 

To add, update or delete a MAPI property, use MsgPropertyCollection.SetValue<T>() method. To delete a property, specify the value argument of null (Nothing in VB.NET).

To read a value of a MAPI property, use one of the three methods:

• MsgPropertyCollection.GetValue<T>(identifier) - fails if the property does not exist or has data type other than specified by the generic type parameter <T>.
• MsgPropertyCollection.GetValue<T>(identifier, defaultValue) - returns defaultValue if the property does not exist, but still fails if data type is mismatched.
• MsgPropertyCollection.TryGetValue<T>(identifier, out value) - safely assigns the value argument and returns true; returns false if the value argument cannot be assigned (either the property does not exists or data type is mismatched).

// load an MsgMessage
// ...

// get the value of 'Subject' property
string subject = message.Properties.GetValue<string>(MsgPropertyTag.Subject);

// set a new 'Subject' property value
message.Properties.SetValue(MsgPropertyTag.Subject, "New subject");

' load an MsgMessage
' ...

' get the value of 'Subject' property
Dim subject As String = message.Properties.GetValue(Of String)(MsgPropertyTag.Subject)

' set a new 'Subject' property value
message.Properties.SetValue(MsgPropertyTag.Subject, "New subject")