如何使用ETag有条件更新Azure表存储实体

Question

如果ETag尚未更改，如何使用Azure表存储实体的ETag更新实体？

例：

   var query = (from ent in cloudservicetable.CreateQuery<CloudServiceTableEntity>()
               where ent.PartitionKey == "test"
               && ent.Status == "creating"
               && ent.Counter> 0
               select ent).AsTableQuery();



    var candidates = query.OrderByDescending(s=>s.Counter).ToList();
    bool found = false;
    while (!found && candidates.Any())
    {
        //Find best candidate
        var candidate = candidates.First();

        //If we can decrement the count with out the entity have been changed
        //it is a acceptable candidate.
        candidate.Counter--;
        var opr = TableOperation.Merge(candidate);
       // cloudservicetable.ExecuteAsync(opr) 
       // How do I only do the merge if the etag have not changed?

       //TODO If changed remove/update the candidate list

    }
    // if found we have a candidate

Answer 1

对于条件合并，您无需执行任何操作。如果在合并操作期间不匹配，您的代码将引发错误（PreCondition-失败Http Status Code 412）ETag。因此，您上面的代码可以正常工作。

对于无条件合并，您需要手动将ETag实体的属性设置为*。