dotnet/Documentation/compatibility/WCF-AddressHeaderCollection-now-throws-an-ArgumentException-if-an-addressHeader-element-is-null.md

##WCF AddressHeaderCollection now throws an ArgumentException if an addressHeader element is null

###Scope Minor ###Version Introduced .NET Framework 4.7.1 ###Source Analyzer Status NotPlanned ###Change Description In .NET Framework 4.7 and earlier versions, there is a bug in one of the public constructors of AddressHeaderCollection where elements of the IEnumerable argument aren’t checked for null. Starting with .NET 4.7.1 the public constructor of AddressHeaderCollection(IEnumerable addressHeaders) now throws an ArgumentException if one of the elements is null. ###Recommended Action: If you run into compatibility issue with this change on .Net 4.7.1 or later version, you may opt-out the change by adding the following configuration in App config file:

<configuration>
    <runtime>
	    <AppContextSwitchOverrides value="Switch.System.ServiceModel.DisableAddressHeaderCollectionValidation=true" />
	</runtime> 
</configuration>

##Affected APIs

• Not detectable via API analysis