如上所述,可以通过Reflection Emit和我下面提供的辅助类来实现。下面的代码还在持续改进中,因此接受它的价值是有限的...“在我的电脑上可以用”。SelectDynamic方法应该被放在一个静态扩展方法类中。
预期的是,由于类型直到运行时才创建,因此您不会得到任何Intellisense。适用于后期绑定数据控件。
public static IQueryable SelectDynamic(this IQueryable source, IEnumerable<string> fieldNames)
{
Dictionary<string, PropertyInfo> sourceProperties = fieldNames.ToDictionary(name => name, name => source.ElementType.GetProperty(name));
Type dynamicType = LinqRuntimeTypeBuilder.GetDynamicType(sourceProperties.Values);
ParameterExpression sourceItem = Expression.Parameter(source.ElementType, "t");
IEnumerable<MemberBinding> bindings = dynamicType.GetFields().Select(p => Expression.Bind(p, Expression.Property(sourceItem, sourceProperties[p.Name]))).OfType<MemberBinding>();
Expression selector = Expression.Lambda(Expression.MemberInit(
Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)), bindings), sourceItem);
return source.Provider.CreateQuery(Expression.Call(typeof(Queryable), "Select", new Type[] { source.ElementType, dynamicType },
Expression.Constant(source), selector));
}
public static class LinqRuntimeTypeBuilder
{
private static readonly ILog log = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
private static AssemblyName assemblyName = new AssemblyName() { Name = "DynamicLinqTypes" };
private static ModuleBuilder moduleBuilder = null;
private static Dictionary<string, Type> builtTypes = new Dictionary<string, Type>();
static LinqRuntimeTypeBuilder()
{
moduleBuilder = Thread.GetDomain().DefineDynamicAssembly(assemblyName, AssemblyBuilderAccess.Run).DefineDynamicModule(assemblyName.Name);
}
private static string GetTypeKey(Dictionary<string, Type> fields)
{
string key = string.Empty;
foreach (var field in fields)
key += field.Key + ";" + field.Value.Name + ";";
return key;
}
public static Type GetDynamicType(Dictionary<string, Type> fields)
{
if (null == fields)
throw new ArgumentNullException("fields");
if (0 == fields.Count)
throw new ArgumentOutOfRangeException("fields", "fields must have at least 1 field definition");
try
{
Monitor.Enter(builtTypes);
string className = GetTypeKey(fields);
if (builtTypes.ContainsKey(className))
return builtTypes[className];
TypeBuilder typeBuilder = moduleBuilder.DefineType(className, TypeAttributes.Public | TypeAttributes.Class | TypeAttributes.Serializable);
foreach (var field in fields)
typeBuilder.DefineField(field.Key, field.Value, FieldAttributes.Public);
builtTypes[className] = typeBuilder.CreateType();
return builtTypes[className];
}
catch (Exception ex)
{
log.Error(ex);
}
finally
{
Monitor.Exit(builtTypes);
}
return null;
}
private static string GetTypeKey(IEnumerable<PropertyInfo> fields)
{
return GetTypeKey(fields.ToDictionary(f => f.Name, f => f.PropertyType));
}
public static Type GetDynamicType(IEnumerable<PropertyInfo> fields)
{
return GetDynamicType(fields.ToDictionary(f => f.Name, f => f.PropertyType));
}
}
Expression.Constant(source)
替换为source.Expression
来解决这个问题。希望这对某些人有所帮助 :) - Connell