IEnumerable和IQueryable的联合

Question

// get cats out of local db
var localDb = db.Categories.Select(c => c.Name);
// get cats out of wcf
var wcf = service.Categories().Select(c => c.CatName);

// create union set
var all = new HashSet<String>(localDb);
all.UnionWith(wcf);

上面的代码工作正常，但是下面的代码引发运行时错误。

var localDb = db.Products.Where(c => c.Category.Equals(name))
                         .Select(p => p.Name);

var wcf = service.Products().Where(c => c.CategoryId == 
                             service.CategoryByName(name).CategoryId)
                            .Select(p => p.ProName);

var all = new HashSet<String>(localDb);
all.UnionWith(wcf);

例外：

An exception of type 'System.ArgumentException' occurred in 
System.Data.Entity.dll but was not handled in user code
Additional information: DbComparisonExpression requires arguments with 
comparable types.

谁能解释为什么第一个有效而第二个无效？

Answer 1

这行：

var localDb = db.Products.Where(c => c.Category.Equals(name))
                .Select(p => p.Name);

尝试将名称（我期望是字符串）与类别对象（我期望不是）进行比较

您无法将两者进行比较，因此会出现错误。

从上一个示例中，我怀疑您想写

var localDb = db.Products.Where(c => c.Category.Name.Equals(name))
                .Select(p => p.Name);