Question

我不确定到底在哪里，但我在某处发现了错误的想法。

我试图在第一个实例中使用linq查询azure存储表。但我无法弄清楚它是如何完成的。通过查看各种来源，我有以下内容：

List<BlogViewModel> blogs = new List<BlogViewModel>();

CloudStorageAccount storageAccount = CloudStorageAccount.Parse(RoleEnvironment.GetConfigurationSettingValue("BlogConnectionString"));
CloudTableClient tableClient = storageAccount.CreateCloudTableClient();
CloudTable blogTable = tableClient.GetTableReference("BlogEntries");

try
{
   TableServiceContext tableServiceContext = tableClient.GetTableServiceContext();
   TableServiceQuery<BlogEntry> query = (from blog in blogTable.CreateQuery<BlogEntry>()
   select blog).AsTableServiceQuery<BlogEntry>(tableServiceContext);
   foreach (BlogEntry blog in query)
   {
      blogs.Add(new BlogViewModel { Body = blog.Body });
   }
}
catch { }

在我搞砸之前，我可能已经把它靠近了。要不然，或者我误解了TableService是什么。以下代码对我有用，但我尝试将其切换为使用Linq。

List<BlogViewModel> blogs = new List<BlogViewModel>();

var storageAccount = CloudStorageAccount.Parse(RoleEnvironment.GetConfigurationSettingValue("BlogConnectionString"));
var tableClient = storageAccount.CreateCloudTableClient();
CloudTable blogTable = tableClient.GetTableReference("BlogEntries");

TableRequestOptions reqOptions = new TableRequestOptions()
{
   MaximumExecutionTime = TimeSpan.FromSeconds(1.5),
   RetryPolicy = new LinearRetry(TimeSpan.FromSeconds(3), 3)
};
List<BlogEntry> lists;

try
{
   var query = new TableQuery<BlogEntry>();
   lists = blogTable.ExecuteQuery(query, reqOptions).ToList();

   foreach (BlogEntry blog in lists)
   {
      blogs.Add(new BlogViewModel { Body = blog.Body });
   }
}
catch { }

我无法在我应该做的任何地方找到一个好的例子。但是从我读过的内容来看，确实建议使用Linq是可能的。任何帮助或指针赞赏。感谢。

轻微更新。以下是我目前在AsTableServiceQuery上的语法错误：

＆＃39; System.Linq.IQueryable＆＃39;不包含＆＃39; AsTableServiceQuery＆＃39;的定义没有扩展方法＆＃39; AsTableServiceQuery＆＃39;接受类型为'System.Linq.IQueryable＆＃39;的第一个参数;可以找到（你错过了使用指令或程序集引用吗？）

然而，我并不认为这反映了真正的问题，我认为我把它放在一起是错误的，只是无法找到一个有效的例子。

Answer 1

Azure存储客户端库的新表服务层中不再需要TableServiceContext。有关此更改的详细信息，请参阅我们的博文Announcing Storage Client Library 2.1 RTM & CTP for Windows Phone。

请确保BlogEntry实现ITableEntity，然后以下代码应该可以正常运行：

List<BlogViewModel> blogs = new List<BlogViewModel>();

CloudTableClient tableClient = storageAccount.CreateCloudTableClient();
CloudTable blogTable = tableClient.GetTableReference("BlogEntries");

try
{
    IEnumerable<BlogEntry> query = (from blog in blogTable.CreateQuery<BlogEntry>()
                                    select blog);
    foreach (BlogEntry blog in query)
    {
        blogs.Add(new BlogViewModel { Body = blog.Body });
    }
}
catch { }

Answer 2

我当前的表存储库执行此操作：

public IQueryable<TEntity> Find(Expression<Func<TEntity, bool>> expression) 
{
    if (IsTableEmpty())
    {
        return Enumerable.Empty<TEntity>().AsQueryable();
    }
    else
    {
        return _cloudTable.CreateQuery<TEntity>().AsQueryable().Where(expression);
    }
}

我的_ cloudTable对应于您的blogTable。

Answer 3

根据之前的回答，我已经创建了支持First, FirstOrDefault, Single and SingleOrDefault的扩展方法：

/// <summary>
/// Provides additional Linq support for the <see cref="TableQuery{TElement}"/> class. 
/// </summary>
public static class LinqToTableQueryExtensions
{
    /// <summary>
    /// Returns the first element in a sequence.
    /// </summary>
    /// <typeparam name="TSource">The type of the elements of source.</typeparam>
    /// <param name="tableQuery">A TableQuery{TSource} to return the first element of</param>
    public static TSource First<TSource>(this TableQuery<TSource> tableQuery) where TSource : ITableEntity
    {
        return ((IEnumerable<TSource>)tableQuery.Take(1)).First();
    }

    /// <summary>
    /// Returns the first element in a sequence that satisfies a specified condition.
    /// </summary>
    /// <typeparam name="TSource">The type of the elements of source.</typeparam>
    /// <param name="tableQuery">A TableQuery{TSource} to return the first element of</param>
    /// <param name="predicate">A function to test an element for a condition.</param>
    public static TSource First<TSource>(this TableQuery<TSource> tableQuery, Expression<Func<TSource, bool>> predicate) where TSource : ITableEntity
    {
        return tableQuery.Where(predicate).Take(1).First();
    }

    /// <summary>
    /// Returns the first element of the sequence or a default value if no such element is found.
    /// </summary>
    /// <typeparam name="TSource">The type of the elements of source.</typeparam>
    /// <param name="tableQuery">A TableQuery{TSource} to return the first element of</param>
    public static TSource FirstOrDefault<TSource>(this TableQuery<TSource> tableQuery) where TSource : ITableEntity
    {
        return ((IEnumerable<TSource>)tableQuery.Take(1)).FirstOrDefault();
    }

    /// <summary>
    /// Returns the first element of the sequence that satisfies a condition or a default value if no such element is found.
    /// </summary>
    /// <typeparam name="TSource">The type of the elements of source.</typeparam>
    /// <param name="tableQuery">A TableQuery{TSource} to return the first element of</param>
    /// <param name="predicate">A function to test an element for a condition.</param>
    public static TSource FirstOrDefault<TSource>(this TableQuery<TSource> tableQuery, Expression<Func<TSource, bool>> predicate) where TSource : ITableEntity
    {
        return tableQuery.Where(predicate).Take(1).FirstOrDefault();
    }

    /// <summary>
    /// Return the only element of a sequence, and throws an exception if there is no exactly one element in the sequence.
    /// </summary>
    /// <typeparam name="TSource">The type of the elements of source.</typeparam>
    /// <param name="tableQuery">A TableQuery{TSource}> to return the single element of</param>
    /// <param name="predicate">A function to test an element for a condition.</param>
    public static TSource Single<TSource>(this TableQuery<TSource> tableQuery, Expression<Func<TSource, bool>> predicate) where TSource : ITableEntity
    {
        // Get 2 and try to get single ^^
        return tableQuery.Where(predicate).Take(2).Single();
    }

    /// <summary>
    /// Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.
    /// </summary>
    /// <typeparam name="TSource">The type of the elements of source.</typeparam>
    /// <param name="tableQuery">A TableQuery{TSource}> to return the single element of</param>
    /// <param name="predicate">A function to test an element for a condition.</param>
    public static TSource SingleOrDefault<TSource>(this TableQuery<TSource> tableQuery, Expression<Func<TSource, bool>> predicate) where TSource : ITableEntity
    {
        // Get 2 and try to get single ^^
        return tableQuery.Where(predicate).Take(2).SingleOrDefault();
    }
}

所以你可以这样使用它：

public class CustomerEntity : TableEntity { public string Email { get; set; } } 
...
var storageAccount = CloudStorageAccount.Parse( "MyStorageAccountConnectionstring");
var tableClient = storageAccount.CreateCloudTableClient();
var table = tableClient.GetTableReference("myTable");

// Linq Query with Where And First
var person = table.CreateQuery<CustomerEntity>()
    .Where(c => c.Email == "Walter1@contoso.com").First();

// Linq query that used the First() Extension method
person = table.CreateQuery<CustomerEntity>()
    .First(c => c.Email == "Walter1@contoso.com");

Answer 4

这里有一些方便的扩展方法来包装它.. （以及ServiceBus自定义属性密钥库的奖励案例）

namespace Microsoft.WindowsAzure.Storage.Table
{
    public static class CloudTableExtensions
    {
        public static TEntity GetTableEntity<TEntity>(this CloudTable cloudTable, BrokeredMessage brokeredMessage, string partitionKeyPropertyName, string rowKeyPropertyName, TableRequestOptions requestOptions = null, OperationContext operationContext = null)
            where TEntity : ITableEntity, new()
        {
            var partitionKey = brokeredMessage.Properties[partitionKeyPropertyName] as string;
            var rowKey = brokeredMessage.Properties[rowKeyPropertyName] as string;
            return GetTableEntity<TEntity>(cloudTable, partitionKey, rowKey, requestOptions, operationContext);
        }

        public static TEntity GetTableEntity<TEntity>(this CloudTable cloudTable, string partitionKey, string rowKey, TableRequestOptions requestOptions = null, OperationContext operationContext = null)
            where TEntity : ITableEntity, new()
        {
            var singleInstanceQuery = (Expression<Func<TEntity, bool>>)(x => x.PartitionKey == partitionKey && x.RowKey == rowKey);
            IEnumerable<TEntity> queryResults = cloudTable.ExecuteQuery(singleInstanceQuery, requestOptions, operationContext);
            return queryResults.SingleOrDefault();
        }

        public static IEnumerable<TEntity> ExecuteQuery<TEntity>(this CloudTable cloudTable, Expression<Func<TEntity, bool>> expression, TableRequestOptions requestOptions = null, OperationContext operationContext = null)
            where TEntity : ITableEntity, new()
        {
            var query = cloudTable.CreateQuery<TEntity>().Where(expression) as TableQuery<TEntity>;
            return cloudTable.ExecuteQuery(query, requestOptions, operationContext);
        }
    }
}

如何使用Linq查询Azure存储表？

4 个答案: