我试图创建一个通用扩展，使用'TryParse'来检查字符串是否为给定类型:
public static bool Is<T>(this string input)
{
T notUsed;
return T.TryParse(input, out notUsed);
}
这将无法编译，因为它无法解析符号TryParse
据我所知，“TryParse”不是任何接口的一部分。
这有可能吗?
更新:
使用以下我想出的答案:
public static bool Is<T>(this string input)
{
try
{
TypeDescriptor.GetConverter(typeof(T)).ConvertFromString(input);
}
catch
{
return false;
}
return true;
}
它工作得很好，但我认为以这种方式使用异常对我来说不合适。
更新2:
修改为传递类型而不是使用泛型:
public static bool Is(this string input, Type targetType)
{
try
{
TypeDescriptor.GetConverter(targetType).ConvertFromString(input);
return true;
}
catch
{
return false;
}
}
利用上面的信息，这就是我开发的东西。它将直接转换对象是可能的，否则它将把对象转换为字符串并调用所需对象类型的TryParse方法。
我将每个方法都缓存在字典中，以减少方法获取负载。
可以测试对象是否可以直接转换为目标类型，这将进一步减少字符串转换部分。但我先不讲这个。
/// <summary>
/// Used to store TryParse converter methods
/// </summary>
private static readonly Dictionary<Type, MethodInfo> TypeConverters = new Dictionary<Type, MethodInfo>();
/// <summary>
/// Attempt to parse the input object to the output type
/// </summary>
/// <typeparam name="T">output type</typeparam>
/// <param name="obj">input object</param>
/// <param name="result">output result on success, default(T) on failure</param>
/// <returns>Success</returns>
public static bool TryParse<T>([CanBeNull] object obj, out T result)
{
result = default(T);
try
{
switch (obj)
{
// don't waste time on null objects
case null: return false;
// if the object is already of type T, just return the value
case T val:
result = val;
return true;
}
// convert the object into type T via string conversion
var input = ((obj as string) ?? obj.ToString()).Trim();
if (string.IsNullOrEmpty(input)) return false;
var type = typeof (T);
Debug.WriteLine($"Info: {nameof(TryParse)}<{type.Name}>({obj.GetType().Name}=\"{input}\")");
if (! TypeConverters.TryGetValue(type, out var method))
{
// get the TryParse method for this type
method = type.GetMethod("TryParse",
new[]
{
typeof (string),
Type.GetType($"{type.FullName}&")
});
if (method is null)
Debug.WriteLine($"FAILED: Cannot get method for {type.Name}.TryParse()");
// store it so we don't have to do this again
TypeConverters.Add(type, method);
}
// have to keep a reference to parameters if you want to get the returned ref value
var parameters = new object[] {input, null};
if ((bool?) method?.Invoke(null, parameters) == true)
{
result = (T) parameters[1];
return true;
}
}
catch (Exception ex)
{
Debug.WriteLine(ex);
}
return false;
}
我使用Charlie Brown的答案作为Json.NET的JObject扩展方法。
public static class JObjectExtension
{
public delegate bool TryParseHandler<T>(string value, out T result);
public static bool TryParsePropertyAs<T>(
this JObject jObject,
string propertyName,
TryParseHandler<T> parser,
out T value)
{
value = default;
if (!jObject.TryGetValue(propertyName, out var jToken)) return false;
if (!parser(jToken.Value<string>(), out T result)) return false;
value = result;
return true;
}
}
利用上面的信息，这就是我开发的东西。它将直接转换对象是可能的，否则它将把对象转换为字符串并调用所需对象类型的TryParse方法。
我将每个方法都缓存在字典中，以减少方法获取负载。
可以测试对象是否可以直接转换为目标类型，这将进一步减少字符串转换部分。但我先不讲这个。
/// <summary>
/// Used to store TryParse converter methods
/// </summary>
private static readonly Dictionary<Type, MethodInfo> TypeConverters = new Dictionary<Type, MethodInfo>();
/// <summary>
/// Attempt to parse the input object to the output type
/// </summary>
/// <typeparam name="T">output type</typeparam>
/// <param name="obj">input object</param>
/// <param name="result">output result on success, default(T) on failure</param>
/// <returns>Success</returns>
public static bool TryParse<T>([CanBeNull] object obj, out T result)
{
result = default(T);
try
{
switch (obj)
{
// don't waste time on null objects
case null: return false;
// if the object is already of type T, just return the value
case T val:
result = val;
return true;
}
// convert the object into type T via string conversion
var input = ((obj as string) ?? obj.ToString()).Trim();
if (string.IsNullOrEmpty(input)) return false;
var type = typeof (T);
Debug.WriteLine($"Info: {nameof(TryParse)}<{type.Name}>({obj.GetType().Name}=\"{input}\")");
if (! TypeConverters.TryGetValue(type, out var method))
{
// get the TryParse method for this type
method = type.GetMethod("TryParse",
new[]
{
typeof (string),
Type.GetType($"{type.FullName}&")
});
if (method is null)
Debug.WriteLine($"FAILED: Cannot get method for {type.Name}.TryParse()");
// store it so we don't have to do this again
TypeConverters.Add(type, method);
}
// have to keep a reference to parameters if you want to get the returned ref value
var parameters = new object[] {input, null};
if ((bool?) method?.Invoke(null, parameters) == true)
{
result = (T) parameters[1];
return true;
}
}
catch (Exception ex)
{
Debug.WriteLine(ex);
}
return false;
}
这为每个泛型类型使用一个静态构造函数，因此它只需要在您第一次对给定类型调用它时执行昂贵的工作。它处理系统名称空间中具有TryParse方法的所有类型。它也适用于这些(结构体)的可空版本，但枚举除外。
public static bool TryParse<t>(this string Value, out t result)
{
return TryParser<t>.TryParse(Value.SafeTrim(), out result);
}
private delegate bool TryParseDelegate<t>(string value, out t result);
private static class TryParser<T>
{
private static TryParseDelegate<T> parser;
// Static constructor:
static TryParser()
{
Type t = typeof(T);
if (t.IsEnum)
AssignClass<T>(GetEnumTryParse<T>());
else if (t == typeof(bool) || t == typeof(bool?))
AssignStruct<bool>(bool.TryParse);
else if (t == typeof(byte) || t == typeof(byte?))
AssignStruct<byte>(byte.TryParse);
else if (t == typeof(short) || t == typeof(short?))
AssignStruct<short>(short.TryParse);
else if (t == typeof(char) || t == typeof(char?))
AssignStruct<char>(char.TryParse);
else if (t == typeof(int) || t == typeof(int?))
AssignStruct<int>(int.TryParse);
else if (t == typeof(long) || t == typeof(long?))
AssignStruct<long>(long.TryParse);
else if (t == typeof(sbyte) || t == typeof(sbyte?))
AssignStruct<sbyte>(sbyte.TryParse);
else if (t == typeof(ushort) || t == typeof(ushort?))
AssignStruct<ushort>(ushort.TryParse);
else if (t == typeof(uint) || t == typeof(uint?))
AssignStruct<uint>(uint.TryParse);
else if (t == typeof(ulong) || t == typeof(ulong?))
AssignStruct<ulong>(ulong.TryParse);
else if (t == typeof(decimal) || t == typeof(decimal?))
AssignStruct<decimal>(decimal.TryParse);
else if (t == typeof(float) || t == typeof(float?))
AssignStruct<float>(float.TryParse);
else if (t == typeof(double) || t == typeof(double?))
AssignStruct<double>(double.TryParse);
else if (t == typeof(DateTime) || t == typeof(DateTime?))
AssignStruct<DateTime>(DateTime.TryParse);
else if (t == typeof(TimeSpan) || t == typeof(TimeSpan?))
AssignStruct<TimeSpan>(TimeSpan.TryParse);
else if (t == typeof(Guid) || t == typeof(Guid?))
AssignStruct<Guid>(Guid.TryParse);
else if (t == typeof(Version))
AssignClass<Version>(Version.TryParse);
}
private static void AssignStruct<t>(TryParseDelegate<t> del)
where t: struct
{
TryParser<t>.parser = del;
if (typeof(t).IsGenericType
&& typeof(t).GetGenericTypeDefinition() == typeof(Nullable<>))
{
return;
}
AssignClass<t?>(TryParseNullable<t>);
}
private static void AssignClass<t>(TryParseDelegate<t> del)
{
TryParser<t>.parser = del;
}
public static bool TryParse(string Value, out T Result)
{
if (parser == null)
{
Result = default(T);
return false;
}
return parser(Value, out Result);
}
}
private static bool TryParseEnum<t>(this string Value, out t result)
{
try
{
object temp = Enum.Parse(typeof(t), Value, true);
if (temp is t)
{
result = (t)temp;
return true;
}
}
catch
{
}
result = default(t);
return false;
}
private static MethodInfo EnumTryParseMethod;
private static TryParseDelegate<t> GetEnumTryParse<t>()
{
Type type = typeof(t);
if (EnumTryParseMethod == null)
{
var methods = typeof(Enum).GetMethods(
BindingFlags.Public | BindingFlags.Static);
foreach (var method in methods)
if (method.Name == "TryParse"
&& method.IsGenericMethodDefinition
&& method.GetParameters().Length == 2
&& method.GetParameters()[0].ParameterType == typeof(string))
{
EnumTryParseMethod = method;
break;
}
}
var result = Delegate.CreateDelegate(
typeof(TryParseDelegate<t>),
EnumTryParseMethod.MakeGenericMethod(type), false)
as TryParseDelegate<t>;
if (result == null)
return TryParseEnum<t>;
else
return result;
}
private static bool TryParseNullable<t>(string Value, out t? Result)
where t: struct
{
t temp;
if (TryParser<t>.TryParse(Value, out temp))
{
Result = temp;
return true;
}
else
{
Result = null;
return false;
}
}
