Ahhh what fun writing unit tests for the REST API, there seems to be quite a few posts about this and I'm afraid I'm going to add another.

I have an Apex REST class that uses a custom class called SAPAccount to hold the details of a JSON message I'm receiving.

global static SAPAccount[] doPost( SAPAccount[] sapsoldtos ){
... logic here...
}

When I try and call this method in my test class I get the error message "Illegal assignment from LIST<SAPAccountREST.SAPAccount> to SAPAccountREST.SAPAccount".

In the test class I'm creating a list of SAPAccounts and then adding to that list.

List<SAPAccountREST.SAPAccount> sapAcctList = new List<SAPAccountREST.SAPAccount>();
String jsonMsg = '';

SAPAccountREST.SAPAccount jsonSapAcct = new SAPAccountREST.SAPAccount();
jsonSapAcct.E1KNA1M_KUNNR = '0000000001';
jsonSapAcct.E1KNA1M_NAME = 'Customer Name';
jsonSapAcct.E1KNA1M_STRAS = 'Street';
jsonSapAcct.E1KNA1M_ORT01 = 'Town';
jsonSapAcct.E1KNA1M_PSTLZ = 'Postal Code';
jsonSapAcct.E1KNA1M_LAND1 = 'Country';
jsonSapAcct.E1KNA1M_STCEG = 'Vat';
		
sapAcctList.add( jsonSapAcct );
jsonMsg = JSON.serialize( jsonSapAcct );

I then try and call the Apex method using the following code which is when of course the error occurs:

SAPAccountREST.SAPAccount results = SAPAccountREST.doPost( sapAcctList );

Would greatly appreciate any help that anyone could provide.