The type 'T' must be a reference type in order to use it as parameter 'T' in the generic type or method 'TableClient.QueryAsync<T> [duplicate]

Question

I'm using 12.0.0-beta.6 of the Azure.Data.Tables nuget package. When I try to invoke TableClient.GetQueryAsync it gives me the error:

"The type 'T' must be a reference type in order to use it as parameter 'T' in the generic type or method 'TableClient.GetEntityAsync(string, string, IEnumerable, CancellationToken)'"

I don't see what I'm doing wrong here. Can anybody advice on how to resolve this error?

  public async Task<IList<T>> QueryAsync<T>(string queryText, CancellationToken cancellationToken) where T : ITableStorageEntity
        {            
            TableClient tableClient = new TableServiceClient("MY_CONNECTION_STRING").GetTableClient("MY_TABLE_NAME");

            var queryResult = await tableClient.QueryAsync<T>(filter: queryText, cancellationToken: cancellationToken);
         
            // prepare and return result list
        }

If I add a class constraint to the declaration, like this:

  public async Task<IList<T>> QueryAsync<T>(string queryText, CancellationToken cancellationToken) where T : ITableStorageEntity

Then these are the errors:

'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'TableClient.QueryAsync(string, int?, IEnumerable, CancellationToken)'

The type 'T' cannot be used as type parameter 'T' in the generic type or method 'TableClient.QueryAsync(string, int?, IEnumerable, CancellationToken)'. There is no implicit reference conversion from 'T' to 'Azure.Data.Tables.ITableEntity'.

Answer 1

Let's look at the doc for that method:

public virtual Azure.AsyncPageable<T> QueryAsync<T> (string filter = default, int? maxPerPage = default, System.Collections.Generic.IEnumerable<string> select = default, System.Threading.CancellationToken cancellationToken = default) where T : class, Azure.Data.Tables.ITableEntity, new();

See that generic type constraint:

where T : class, Azure.Data.Tables.ITableEntity, new();

That means that any T you pass in must be a class, must implement ITableEntity, and must have a parameterless constructor.

However, your method doesn't enforce this. You only require that T implements ITableStorageEntity. Your method could theoretically accept something which implements ITableStorageEntity but isn't a class, or doesn't have a parameterless constructor, and pass it to Azure's QueryAsync<T>, and then what? You've broken the rules!

Your method needs to have the same generic type constraints as QueryAsync<T>, or tighter:

public async Task<IList<T>> QueryAsync<T>(string queryText, CancellationToken cancellationToken)
    where T : class, ITableStorageEntity, new()