Word Properties Automation

[=?Utf-8?B?UHJvYmk=?=]

I´m trying to do some Word Automation in C# in the Custom Properties
so far found very few ways to do that

I made the following :
Type Prop = Type.GetTypeFro mProgID("Word.C ustopParopertie s");
System.Object obj = Activator.Creat eInstance(Prop) ;
Microsoft.Offic e.Interop.Word. CustomPropertie s CustomProp = obj as
Microsoft.Offic e.Interop.Word. CustomPropertie s;
foreach (Microsoft.Offi ce.Interop.Word .CustomProperty property in CustomProp)
{ if (property.Name == valfind) property.Value = valreplace; }

So far I´ve been getting the following error

System.Argument NullException: Value cannot be null.
Parameter name: type

Source Error:
Line 53:
Line 54: Type Prop =
Type.GetTypeFro mProgID("Word.C ustopParopertie s");
Line 55: System.Object obj = Activator.Creat eInstance(Prop) ;
Line 56:
Line 57: Microsoft.Offic e.Interop.Word. CustomPropertie s
CustomProp = obj as Microsoft.Offic e.Interop.Word. CustomPropertie s;

I understand I dont have the ProgID right but have no idea how to do this,
I´m quite new in C# and all
Even dont know if this is the best place for my question

Any help I´ll be thankful