Compile error using Azure Storage Client library 3.0 method executequery<T> where T is tableentity
azure-storage, azure-table-storage, c#-4.0
Solution
So turns out that if a method provides a constraint on type, that constraint must be also forwarded by any callers of that type.
I didn't know that.
In this case the definition for Table.ExecuteQuery looks like the following
public IEnumerable<TElement> ExecuteQuery<TElement>(TableQuery<TElement> query,
TableRequestOptions requestOptions = null,
OperationContext operationContext = null)
where TElement : ITableEntity, new();
Therefore adding new() to my constraints for T fixes the issue.
So the final method declaration looks like
internal static IEnumerable<T> GetEntitiesWithCommaSeparatedRowKeys<T>(string tableName,
string partitionKey,
string commaDelimitedStringOfRowKeys)
where T : TableEntity , new() //This is new (pun intended :))
Found a related issue in one of the related links that showed up for this question.
I'm guessing the compiler could always look up the type constraints from when I actually make the call, but since TableEntity is public (and not sealed) I guess it could end up being a runtime issue.
Also interesting to note that my method is marked internal which should really enable the compiler to check against callers within the library.
Anyways, learnt something new :).
Problem
I must be missing something obvious but the following fails with a compile error: ``` internal static IEnumerable<T> GetEntitiesWithCommaSeparatedRowKeys<T>( string tableName, string partitionKey, string commaDelimitedStringOfRowKeys) where T: TableEntity { .... TableQuery<T> entitiesQuery = new TableQuery<T>().Where( TableQuery.CombineFilters( TableQuery.GenerateFilterCondition("PartitionKey", QueryComparisons.Equal, partitionKey), TableOperators.And, AzureHelper.GetFilterConditionForCommaDelimitedStringOfRowKeys(commaDelimitedStringOfRowKeys) )); // compile error on this line IEnumerable<T> entities = table.ExecuteQuery<T>(entitiesQuery); ... } ``` The error I get is: ``` 'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'TElement' in the generic type or method 'Microsoft.WindowsAzure.Storage.Table.CloudTable.ExecuteQuery<TElement> (Microsoft.WindowsAzure.Storage.Table.TableQuery<TElement>, Microsoft.WindowsAzure.Storage.Table.TableRequestOptions, Microsoft.WindowsAzure.Storage.OperationContext)' ``` TableEntity clearly has a public parameterless constructor and is non-abstract. The following is the object from metadata info when I hit F12 on TableEntity (just to ensure its resolving the TableEntity type correctly). ``` namespace Microsoft.WindowsAzure.Storage.Table { public class TableEntity : ITableEntity { // Summary: // Initializes a new instance of the Microsoft.WindowsAzure.Storage.Table.TableEntity // class. public TableEntity(); ... } } ``` Any ideas anyone? FYI I'm using Azure Client library 3.0.1.0. UPDATE: Added linked issue which solved a similar problem