getNextInvoiceNo.fs

let rec getNextInvoiceNo (table:CloudTable) operatorID = task {
    let rowKey = "invoiceno"
    let query = TableOperation.Retrieve(operatorID, rowKey)
    let! r = table.ExecuteAsync(query)
    if r.HttpStatusCode <> 200 || isNull r.Result then
        let entity = DynamicTableEntity()
        entity.PartitionKey <- operatorID
        entity.RowKey <- rowKey
        let no = 1
        let operation = TableOperation.Insert entity
        entity.PartitionKey <- operatorID
        entity.RowKey <- rowKey
        entity.Properties.["InvoiceNo"] <- EntityProperty.GeneratePropertyForInt(System.Nullable(no + 1))
        let! _ =  table.ExecuteAsync operation
        return no
    else
        let entity = r.Result :?> DynamicTableEntity
        let no = getIntProperty "InvoiceNo" entity

        let operation = TableOperation.Replace entity
        entity.Properties.["InvoiceNo"] <- EntityProperty.GeneratePropertyForInt(System.Nullable(no + 1))
        try
            let! r =  table.ExecuteAsync operation
            return no
        with
        | _ -> return! getNextInvoiceNo table operatorID 
}