Merge pull request #54112 from rodrigoaatmicrosoft/patch-4

Adding Log documentation to C# UDF

Author: ktoliver

articles/stream-analytics/stream-analytics-edge-csharp-udf-methods.md

@@ -136,6 +136,43 @@ Expand the **User-Defined Code Configuration** section, and fill out the configu
    |Custom Code Assembly Source|Existing assembly packages from the cloud|
    |Custom Code Assembly Source|UserCustomCode.zip|
 
+## User logging
+The logging mechanism allows you to capture custom information while a job is running. You can use log data to debug or assess the correctness of the custom code in real time.
+
+The `StreamingContext` class lets you publish diagnostic information using the `StreamingDiagnostics.WriteError` function. The code below shows the interface exposed by Azure Stream Analytics.
+
+```csharp
+public abstract class StreamingContext
+{
+    public abstract StreamingDiagnostics Diagnostics { get; }
+}
+
+public abstract class StreamingDiagnostics
+{
+    public abstract void WriteError(string briefMessage, string detailedMessage);
+}
+```
+
+`StreamingContext` is passed as an input parameter to the UDF method and can be used within the UDF to publish custom log information. In the example below, `MyUdfMethod` defines a **data** input, which is provided by the query, and a **context** input as the `StreamingContext`, provided by the runtime engine. 
+
+```csharp
+public static long MyUdfMethod(long data, StreamingContext context)
+{
+    // write log
+    context.Diagnostics.WriteError("User Log", "This is a log message");
+    
+    return data;
+}
+```
+
+The `StreamingContext` value doesn't need to be passed in by the SQL query. Azure Stream Analytics provides a context object automatically if an input parameter is present. The use of the `MyUdfMethod` does not change, as shown in the following query:
+
+```sql
+SELECT udf.MyUdfMethod(input.value) as udfValue FROM input
+```
+
+You can access log messages through the [diagnostic logs](data-errors.md).
+
 ## Limitations
 The UDF preview currently has the following limitations: