Return Case Number After Insert a new Case

Question

I've created an Apex Class which inserts new Case. On the tutorial it only returns the ID. I've checked and researched on how to return the CaseNumber value. But it gives me an error on when called on PostMan

APEX Class

    Global static id caseinsert(string Status, string Origin,string SocialMediaAccount, String CustomerID
    ,string Category, string SubCategory, string SConcern,String NatureOfConcern){
   
    Case css = new case(
    Status = Status,
    Origin = Origin,
    Social_Media_Account__c = SocialMediaAccount,
    Billing_Customer_ID__c = CustomerID,
    Case_Category__c =  Category,
    Case_Sub_Category__c = SubCategory,
    Specific_Concern__c= SConcern,
    Nature_Of_Concern__c =  NatureOfConcern

    );
    string casenum;
    insert css;
    css =[select id,CaseNumber from Case where id=:css.id];  
       
    return css.CaseNumber;
    }

Error on Postman

[
    {
        "errorCode": "APEX_ERROR",
        "message": "System.StringException: Invalid id: 00107329\n\nClass.CustomRestAPI.caseinsert: line 30, column 1"
    }
]

Answer 1

You have declared your method to return an Id:

Global static id caseinsert(...)

but you are returning a value that is not an Id, but a String:

return css.CaseNumber;

Id values are checked for validity on creation. Change your method signature to reflect its actual return value, String.