Question

Windows Azure表存储does not support 十进制数据类型。

suggested workaround是使用自定义属性将decimal属性序列化为字符串：

[EntityDataType(PrimitiveTypeKind.String)]
public decimal Quantity { get; set; }

如何实现此EntityDataType自定义属性，以便可以从Windows Azure Tables存储和检索小数属性？

Answer 1

覆盖基类中的ReadEntity和WriteEntity对此有好处。每次检索实体时都没有必要写EntityResolver。

public class CustomTableEntity : TableEntity
{
    public override void ReadEntity(IDictionary<string, EntityProperty> properties, OperationContext operationContext)
    {
        base.ReadEntity(properties, operationContext);

        foreach (var thisProperty in
            GetType().GetProperties().Where(thisProperty =>
                thisProperty.GetType() != typeof(string) &&
                properties.ContainsKey(thisProperty.Name) &&
                properties[thisProperty.Name].PropertyType == EdmType.String))
        {
            var parse = thisProperty.PropertyType.GetMethods().SingleOrDefault(m =>
                m.Name == "Parse" &&
                m.GetParameters().Length == 1 &&
                m.GetParameters()[0].ParameterType == typeof(string));

            var value = parse != null ?
                parse.Invoke(thisProperty, new object[] { properties[thisProperty.Name].StringValue }) :
                Convert.ChangeType(properties[thisProperty.Name].PropertyAsObject, thisProperty.PropertyType);

            thisProperty.SetValue(this, value);
        }
    }

    public override IDictionary<string, EntityProperty> WriteEntity(OperationContext operationContext)
    {
        var properties = base.WriteEntity(operationContext);

        foreach (var thisProperty in
            GetType().GetProperties().Where(thisProperty =>
                !properties.ContainsKey(thisProperty.Name) &&
                typeof(TableEntity).GetProperties().All(p => p.Name != thisProperty.Name)))
        {
            var value = thisProperty.GetValue(this);
            if (value != null)
            {
                properties.Add(thisProperty.Name, new EntityProperty(value.ToString()));
            }
        }

        return properties;
    }
}

使用时，只需使实体从CustomTableEntity扩展，插入或检索实体时它将是透明的。它支持DateTime，TimeSpan，decimal以及具有Parse方法或实现IConvertible接口的类型。

Answer 2

您可以覆盖TableEntity中的WriteEntity方法并使用EntityResolver

public class CustomTableEntity : TableEntity
{
    private const string DecimalPrefix = "D_";

    public override IDictionary<string, EntityProperty> WriteEntity(OperationContext operationContext)
    {
        var entityProperties = base.WriteEntity(operationContext);
        var objectProperties = GetType().GetProperties();

        foreach (var item in objectProperties.Where(f => f.PropertyType == typeof (decimal)))
        {
            entityProperties.Add(DecimalPrefix + item.Name, new EntityProperty(item.GetValue(this, null).ToString()));
        }

        return entityProperties;
    }
}

我们将使用的实体

public class MyEntity : CustomTableEntity
{
    public string MyProperty { get; set; }

    public decimal MyDecimalProperty1 { get; set; }
    public decimal MyDecimalProperty2 { get; set; }
}

用法，其中包括创建表格/插入/撤消

#region connection

CloudStorageAccount account = CloudStorageAccount.DevelopmentStorageAccount;
CloudTableClient client = account.CreateCloudTableClient();
CloudTable table = client.GetTableReference("mytable");
table.CreateIfNotExists();

#endregion


const string decimalPrefix = "D_";

const string partitionKey = "BlaBlaBla";
string rowKey = DateTime.Now.ToString("yyyyMMddHHmmss");


#region Insert

var entity = new MyEntity
    {
        PartitionKey = partitionKey,
        RowKey = rowKey,
        MyProperty = "Test",
        MyDecimalProperty1 = (decimal) 1.2,
        MyDecimalProperty2 = (decimal) 3.45
    };

TableOperation insertOperation = TableOperation.Insert(entity);
table.Execute(insertOperation);

#endregion



#region Retrieve

EntityResolver<MyEntity> myEntityResolver = (pk, rk, ts, props, etag) =>
    {
        var resolvedEntity = new MyEntity {PartitionKey = pk, RowKey = rk, Timestamp = ts, ETag = etag};

        foreach (var item in props.Where(p => p.Key.StartsWith(decimalPrefix)))
        {
            string realPropertyName = item.Key.Substring(decimalPrefix.Length);
            System.Reflection.PropertyInfo propertyInfo = resolvedEntity.GetType().GetProperty(realPropertyName);
            propertyInfo.SetValue(resolvedEntity, Convert.ChangeType(item.Value.StringValue, propertyInfo.PropertyType), null);

        }

        resolvedEntity.ReadEntity(props, null);

        return resolvedEntity;
    };

TableOperation retrieveOperation = TableOperation.Retrieve(partitionKey, rowKey, myEntityResolver);
TableResult retrievedResult = table.Execute(retrieveOperation);
var myRetrievedEntity = retrievedResult.Result as MyEntity;

// myRetrievedEntity.Dump(); 

#endregion

Answer 3

您是否尝试过使用Lokad.Cloud FatEntities产品？

我认为他们只是使用二进制序列化器来处理要存储在表中的整个对象。看看“对象到云映射器”项目也是值得的：

https://github.com/Lokad/lokad-cloud

Answer 4

@EUYUIL已经提出了一个很好的通用解决方案，我已经习惯了很好的效果，但是当他的回答表明它在使用Nullable类型时会失败。

       // Get the underlying types 'Parse' method
       if (curType.IsGenericType && curType.GetGenericTypeDefinition() == typeof(Nullable<>))
       {
            curType = Nullable.GetUnderlyingType(curType);
       }

如果它帮助任何人，在foreach内部的ReadEntity覆盖方法的内容。可能有更好的方法来写这个，但为了便于说明，这样做。

        var curType = thisProperty.PropertyType;

        // Get the underlying types 'Parse' method
        if (curType.IsGenericType && curType.GetGenericTypeDefinition() == typeof(Nullable<>))
        {
            curType = Nullable.GetUnderlyingType(curType);
        }

        var parse = curType.GetMethods().SingleOrDefault(m =>
            m.Name == "Parse" &&
            m.GetParameters().Length == 1 &&
            m.GetParameters()[0].ParameterType == typeof(string));

        var value = parse != null ?
            parse.Invoke(thisProperty, new object[] { properties[thisProperty.Name].StringValue }) :
                  Convert.ChangeType(properties[thisProperty.Name].PropertyAsObject, thisProperty.PropertyType);


        thisProperty.SetValue(this, value);

在Azure表中存储十进制数据类型

4 个答案: