How to fix warning CS8602: Dereference of a possibly null reference?

Question

How to avoid this Warning

warning CS8602: Dereference of a possibly null reference

The code:

public async Task<IEnumerable<T>> ExecuteQuery<T>(string commandText, string ctx, CancellationToken cancellationToken, CommandType commandType = CommandType.Text, object? paramSet = null)
{
    var dtsrc = CheckConnectionToDbServer(ctx, cancellationToken);
    //if (dtsrc == null) { _logger.LogTrace($"{methodName}: No datasource!"); return 0; }
    var con = dtsrc.OpenConnection();//<---? 
    IEnumerable<T> result2 = await con.QueryAsync<T>(commandText, paramSet, commandType: commandType).ConfigureAwait(false);
    return result2;
}

-- If dtsrc is null then con is null too and I cannot return Task<IEnumerable<T>> from the function. How to fix it?

And the 2nd function: how to return null or something like in case of no connection?

public async Task<T> ExecuteScalarQuery<T>(string commandText, string ctx, CancellationToken cancellationToken, CommandType commandType = CommandType.Text, object? paramSet = null)
{
    string methodName = nameof(ExecuteScalarQuery);
    _logger.LogTrace($"{methodName}: {ctx}");
    var dtsrc = CheckConnectionToDbServer(ctx, cancellationToken);

    if (dtsrc == null)
    {
        _logger.LogTrace($"{methodName}: No datasource!");
        //return // -<---???
    }
    var con = dtsrc.OpenConnection();
    return await con.QuerySingleAsync<T>(commandText, paramSet, commandType: commandType).ConfigureAwait(false);
}

Does this answer your question? [In C# 8, why does type inference on new expressions result in nullable references?](https://stackoverflow.com/questions/62685673/in-c-sharp-8-why-does-type-inference-on-new-expressions-result-in-nullable-refe) — vernou, Jun 14 '23 at 10:31
This code is suspicious - connections are *not* meant to be cached, they should be closed as soon as possible. That's why they're always created in a `using` block. ADO.NET's connection pooling takes care of removing the cost of closing and opening connections, by resetting existing ones and reusing them. If `QueryAsync` comes from Dapper, there's no need to call `Open` either, it's done automatically. — Panagiotis Kanavos, Jun 14 '23 at 10:47
The Dapper documentation shows how it should be used. This code should be replaced with `using(var con=new SqlConnection(conn_str){ var results=await con.QueryAsync(...); ....}`. — Panagiotis Kanavos, Jun 14 '23 at 10:49
But what if connection is null? I need to return something from my function — ZedZip, Jun 14 '23 at 10:59
`new SqlConnection` is never null. The far bigger problem is the multiple connection leaks that will cause this code to freeze production. Use ADO.NET and Dapper correctly and the problems go away — Panagiotis Kanavos, Jun 14 '23 at 11:01
You wouldn't be getting a warning if `CheckConnectionToDbServer` didn't return nulls either. Which means that either it's an external method that wasn't compiled with nullability checks, or it returns `DbConnection?`, which should be happening. You can bypass this by actually checking for nulls, but *why not remove the bugs entirely* ? — Panagiotis Kanavos, Jun 14 '23 at 11:03
Please check my edited start post. I have added the 2nd function. How to re-write it? — ZedZip, Jun 14 '23 at 11:10

jason.kaisersmith · Accepted Answer · 2023-06-14T10:41:12.943

2

You are on the right path with the part you commented out. Just return an empty Enumerable.

if (dtsrc == null) 
{ 
    _logger.LogTrace($"{methodName}: No datasource!"); 
    return Task.FromResult(Enumerable.Empty<T>());
}

Alternatively you could

Throw an exception or Allow your method to return null, and check for null in your calling method and handle it there.

edited Jun 14 '23 at 10:41

answered Jun 14 '23 at 10:30

jason.kaisersmith

8,712
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I try this: return Enumerable.Empty(); CS0117: IEnumerable does not contain a definition for Empty – ZedZip Jun 14 '23 at 10:35
Ah Yes, you are running async and returning a Task. I have updated it. It should work now. – jason.kaisersmith Jun 14 '23 at 10:41
1

There's no need for `Task.FromResult` as this is an `async` method. – Panagiotis Kanavos Jun 14 '23 at 10:44
@ZedZip [Enumerable.Empty](https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.empty?view=net-7.0) is available in all .NET versions. If you get an error it's probably because you missed a `using System.Linq;` – Panagiotis Kanavos Jun 14 '23 at 10:45
Yes, I have added System.Linq – ZedZip Jun 14 '23 at 10:47
How to return null in case of : public async Task ExecuteScalarQuery(...) ? – ZedZip Jun 14 '23 at 10:49
@ZedZip why are you trying to wrap Dapper's methods? They work fine the way they are. They have simpler syntax than what you're trying to do, better connection management. There's `conn.ExecuteScalar(sql)` already – Panagiotis Kanavos Jun 14 '23 at 10:52
@PanagiotisKanavos you are right, but I mean a situation like in my 1st post: when connection is null , the query cannot be executed so I need to return something from the function – ZedZip Jun 14 '23 at 10:56
I need something like this if (dtsrc == null) { _logger.LogTrace($"{methodName}: No datasource!"); return Task.FromResult(Enumerable.Empty()); // <- and not Enumerable but something like Task } – ZedZip Jun 14 '23 at 10:57
@ZedZip the connection is never null when you create safely - in a `using` block. You're trying to cover up Dapper's safe code with custom *un*safe code – Panagiotis Kanavos Jun 14 '23 at 10:59
Your return type for the method is "Task>", so you can't return anything which does not confirm to this! – jason.kaisersmith Jun 14 '23 at 11:00
@jason.kaisersmith yes, in this function all is ok. I have the 2nd one where public async Task ExecuteScalarQuery(...) – ZedZip Jun 14 '23 at 11:01

How to fix warning CS8602: Dereference of a possibly null reference?

1 Answers1