Question

Below is my code . Please review it .

 1. bool isUnavailable = db.Deploys.Where(p => 
     p.HostEnvironmentId == Guid.Parse(host.ID) &&
     p.Status == (int)DeployStatus.Deploying).AsEnumerable().Any();

This one works.

The following statements doesn’t work.

2. bool isUnavailable = db.Deploys.Where(p => 
    p.HostEnvironmentId == Guid.Parse(host.ID) &&
    p.Status == (int)DeployStatus.Deploying).Any();//Error 

The Exception is

    An exception of type 'System.NotSupportedException' occurred in 
Microsoft.Data.Services.Client.DLL but was not handled in user code
    
    Additional information: The method 'Any' is not supported.


3. bool isUnavailable = db.Deploys.Where(p => 
        p.HostEnvironmentId.ToString() == host.ID &&
        p.Status == (int)DeployStatus.Deploying).AsEnumerable().Any();//Error

The Exception is

    An exception of type 'System.NotSupportedException' occurred in 
      Microsoft.Data.Services.Client.DLL but was not handled in user code
      Additional information: The expression (([10007].HostEnvironmentId.ToString() == 
"b7db845b-cec4-49af-8f4b-b419a4e44331") And ([10007].Status == 90)) is not supported.

The Deploys Class is the model which is built in the client proxy class of WCF Data service. I was using "add service reference" to create WCF client proxy class.

But as to the Generic List,
Supposed below code. it will works fine.

4.bool b=servers.Where(d => 
   d.status == (int)Enums.ServerStatus.Deploying ||
   d.status == int)Enums.ServerStatus.Unavailable).Any();

My question is

Why same way used in different Class got different result .(See the method 2 and method 4).

Why 2 and 3 don’t work.

Hope someone can help me . Thanks

Answer 1

LINQ has a concept of ‘providers’. When working with LINQ over different data sources, different things need to happen for identical LINQ queries depending on the data source.

For example, when you want to use LINQ to query a database, the LINQ query needs to be converted to an SQL query. When the data source is OData, the query needs to be converted to a URL. There are different providers for each, and each provider supports a different subset of LINQ operators and other language constructs. LINQ-to-SQL, Entity Framework and LINQ-to-NHibernate are three popular LINQ providers for database access.

In your case, you are using WCF Data Services which includes a LINQ provider for OData. Since in OData there is no way to express the .Any() LINQ operator, attempting to use it in a query with that provider throws an exception. By using .AsEnumerable() you’re essentially saying to stop using the OData LINQ provider at that point and start using the LINQ-to-Objects provider (which isn’t technically a provider, but conceptually you can think of it as one). That means only what comes before .AsEnumerable() will be converted to an OData query, causing to retrieve all the Deploy entities that match the .Where(), and after they have all been transferred to the client, the client will perform the .Any() by checking the number of Deploy entities it has received. This of course is bad if there are many such entities, it will cause unwanted transfer of data over the network when all you wanted is the server side (database probably) to check if there are any. Unfortunately, .Any() is not supported by OData 1.0 (I don’t know about OData 2.0).

Also, OData may not support .ToString() either. You may need to compare the Guid structures directly, i.e. create a local variable that contains the GUID value you want to compare:

var g = Guid.Parse("b7db845b-cec4-49af-8f4b-b419a4e44331")`

And then in the query compare the GUIDs like so:

x.HostedEnvironment == g