Getting “The connection does not support MultipleActiveResultSets” in a ForEach with async-await

Question

I have the following code using Dapper.SimpleCRUD :

var test = new FallEnvironmentalCondition[] {
    new FallEnvironmentalCondition {Id=40,FallId=3,Environm         


        

Answer 1

MARS has some limitations and also a non-zero overhead. You can use the following helpers to make the updates sequential:

public static async Task WhenAllOneByOne(this IEnumerable source, Func process)
{
    foreach (var item in source)
        await process(item);
}

public static async Task> WhenAllOneByOne(this IEnumerable source, Func> transform)
{
    var results = new List();

    foreach (var item in source)
        results.Add(await transform(item));

    return results;
    // I would use yield return but unfortunately it is not supported in async methods
}

So your example would turn into

await test.WhenAllOneByOne(conn.UpdateAsync);

I usually call the second helper instead of Task.WhenAll, as follows:

await Task.WhenAll(source.Select(transform)); // not MARS-safe
await source.WhenAllOneByOne(transform); // MARS-safe