[英]Populating nullable type from SqlDataReader using Reflection.Emit
嘗試使用reflection.emit設置任何可為空的數據類型的值時,不會填充可為空的數據類型的值。
例如
public class Employee
{
public int Id { get;set;}
public string? Name {get;set;}
public DateTime? Joined {get;set;}
public bool? IsManager {get;set;}
}
這是我的Reflection.emit代碼,用於填充sql數據閱讀器:
private static readonly MethodInfo GetValueMethod = typeof(SqlDataReader).GetMethod("get_Item", new Type[] { typeof(int) });
private static readonly MethodInfo IsDBNullMethod = typeof(SqlDataReader).GetMethod("IsDBNull", new Type[] { typeof(int) });
DynamicMethod method = new DynamicMethod("DynamicCreateMapping", typeof(T), new Type[] { typeof(IDataRecord) }, typeof(T), true);
ILGenerator generator = method.GetILGenerator();
LocalBuilder result = generator.DeclareLocal(typeof(T));
generator.Emit(OpCodes.Newobj, typeof(T).GetConstructor(Type.EmptyTypes));
generator.Emit(OpCodes.Stloc, result);
for (int i = 0; i < reader.FieldCount; i++)
{
PropertyInfo propertyInfo = typeof(T).GetProperty(reader.GetName(i));
Label endIfLabel = generator.DefineLabel();
if (propertyInfo != null && propertyInfo.GetSetMethod() != null)
{
generator.Emit(OpCodes.Ldarg_0);
generator.Emit(OpCodes.Ldc_I4, i);
generator.Emit(OpCodes.Callvirt, IsDBNullMethod);
generator.Emit(OpCodes.Brtrue, endIfLabel);
generator.Emit(OpCodes.Ldloc, result);
generator.Emit(OpCodes.Ldarg_0);
generator.Emit(OpCodes.Ldc_I4, i);
generator.Emit(OpCodes.Callvirt, GetValueMethod);
generator.Emit(OpCodes.Unbox_Any, reader.GetFieldType(i));
generator.Emit(OpCodes.Callvirt, propertyInfo.GetSetMethod());
generator.MarkLabel(endIfLabel);
}
}
generator.Emit(OpCodes.Ldloc, result);
generator.Emit(OpCodes.Ret);
我知道以下行中存在問題,但不確定如何解決:
generator.Emit(OpCodes.Unbox_Any, reader.GetFieldType(i));
我嘗試設置,但使用以下命令失敗:
generator.Emit(OpCodes.Unbox_Any, propertyInfo.PropertyType);
那么如何從類而不是從sql數據讀取器匹配數據類型呢?
我斷了一段時間后才修好它。 感謝鏈接[ http://tillitsonpaper.blogspot.co.uk/2011/05/mapping-data-reader-to-object.html][1] 。 我修改了很少的代碼並將其粘貼到此處,以便對那些對反射知之甚少的新手有所幫助。
我添加了條件來檢查布爾值和枚舉類型:
if (propertyInfo.PropertyType.IsEnum || propertyInfo.PropertyType == typeof(Boolean))
{
generator.Emit(OpCodes.Unbox_Any, reader.GetFieldType(i));
}
else
{
generator.Emit(OpCodes.Call, GetConverterMethod(propertyInfo.PropertyType));
}
上面的else部分將處理任何類型,包括可空類型,GetConverterMethod實現如下所示:
private MethodInfo GetConverterMethod(Type type)
{
switch (type.Name.ToUpper())
{
case "INT16":
return CreateConverterMethodInfo("ToInt16");
case "INT32":
return CreateConverterMethodInfo("ToInt32");
case "INT64":
return CreateConverterMethodInfo("ToInt64");
case "SINGLE":
return CreateConverterMethodInfo("ToSingle");
case "BOOLEAN":
return CreateConverterMethodInfo("ToBoolean");
case "STRING":
return CreateConverterMethodInfo("ToString");
case "DATETIME":
return CreateConverterMethodInfo("ToDateTime");
case "DECIMAL":
return CreateConverterMethodInfo("ToDecimal");
case "DOUBLE":
return CreateConverterMethodInfo("ToDouble");
case "GUID":
return CreateConverterMethodInfo("ToGuid");
case "BYTE[]":
return CreateConverterMethodInfo("ToBytes");
case "BYTE":
return CreateConverterMethodInfo("ToByte");
case "NULLABLE`1":
{
if (type == typeof(DateTime?))
{
return CreateConverterMethodInfo("ToDateTimeNull");
}
else if (type == typeof(Int32?))
{
return CreateConverterMethodInfo("ToInt32Null");
}
else if (type == typeof(Boolean?))
{
return CreateConverterMethodInfo("ToBooleanNull");
}
break;
}
}
return null;
}
private MethodInfo CreateConverterMethodInfo(string method)
{
return typeof(Converter).GetMethod(method, new Type[] { typeof(object) });
}
轉換器類代碼如下:
public class Converter
{
public static Int16 ToInt16(object value)
{
return ChangeType<Int16>(value);
}
public static Int32 ToInt32(object value)
{
return ChangeType<Int32>(value);
}
public static Int64 ToInt64(object value)
{
return ChangeType<Int64>(value);
}
public static Single ToSingle(object value)
{
return ChangeType<Single>(value);
}
public static Boolean ToBoolean(object value)
{
return ChangeType<Boolean>(value);
}
public static System.String ToString(object value)
{
return ChangeType<System.String>(value);
}
public static DateTime ToDateTime(object value)
{
return ChangeType<DateTime>(value);
}
public static Decimal ToDecimal(object value)
{
return ChangeType<Decimal>(value);
}
public static Double ToDouble(object value)
{
return ChangeType<Double>(value);
}
public static Guid ToGuid(object value)
{
return ChangeType<Guid>(value);
}
public static Byte ToByte(object value)
{
return ChangeType<Byte>(value);
}
public static Byte[] ToBytes(object value)
{
return ChangeType<Byte[]>(value);
}
public static DateTime? ToDateTimeNull(object value)
{
return ChangeType<DateTime?>(value);
}
public static System.Int32? ToInt32Null(object value)
{
return ChangeType<Int32?>(value);
}
public static Boolean? ToBooleanNull(object value)
{
return ChangeType<Boolean?>(value);
}
private static T ChangeType<T>(object value)
{
var t = typeof(T);
if (t.IsGenericType && t.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
{
if (value == null)
{
return default(T);
}
t = Nullable.GetUnderlyingType(t); ;
}
return (T)Convert.ChangeType(value, t);
}
}
可能不是很優雅的做法,但這也適用於任何可為null的類型。 希望這對那些努力使Nullable類型與Reflection.Emit一起使用的人有所幫助。
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