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;
    }
}

当前回答

受到Charlie Brown在这里发布的解决方案的启发,我使用反射创建了一个通用的TryParse,可以选择输出解析后的值:

/// <summary>
/// Tries to convert the specified string representation of a logical value to
/// its type T equivalent. A return value indicates whether the conversion
/// succeeded or failed.
/// </summary>
/// <typeparam name="T">The type to try and convert to.</typeparam>
/// <param name="value">A string containing the value to try and convert.</param>
/// <param name="result">If the conversion was successful, the converted value of type T.</param>
/// <returns>If value was converted successfully, true; otherwise false.</returns>
public static bool TryParse<T>(string value, out T result) where T : struct {
    var tryParseMethod = typeof(T).GetMethod("TryParse", BindingFlags.Static | BindingFlags.Public, null, new [] { typeof(string), typeof(T).MakeByRefType() }, null);
    var parameters = new object[] { value, null };

    var retVal = (bool)tryParseMethod.Invoke(null, parameters);

    result = (T)parameters[1];
    return retVal;
}

/// <summary>
/// Tries to convert the specified string representation of a logical value to
/// its type T equivalent. A return value indicates whether the conversion
/// succeeded or failed.
/// </summary>
/// <typeparam name="T">The type to try and convert to.</typeparam>
/// <param name="value">A string containing the value to try and convert.</param>
/// <returns>If value was converted successfully, true; otherwise false.</returns>
public static bool TryParse<T>(string value) where T : struct {
    T throwaway;
    var retVal = TryParse(value, out throwaway);
    return retVal;
}

它可以这样被称为:

string input = "123";
decimal myDecimal;

bool myIntSuccess = TryParse<int>(input);
bool myDecimalSuccess = TryParse<decimal>(input, out myDecimal);

更新: 也感谢我非常喜欢的YotaXP的解决方案,我创建了一个不使用扩展方法但仍然有一个单例的版本,最大限度地减少了反射的需要:

/// <summary>
/// Provides some extra parsing functionality for value types.
/// </summary>
/// <typeparam name="T">The value type T to operate on.</typeparam>
public static class TryParseHelper<T> where T : struct {
    private delegate bool TryParseFunc(string str, out T result);

    private static TryParseFunc tryParseFuncCached;

    private static TryParseFunc tryParseCached {
        get {
            return tryParseFuncCached ?? (tryParseFuncCached = Delegate.CreateDelegate(typeof(TryParseFunc), typeof(T), "TryParse") as TryParseFunc);
        }
    }

    /// <summary>
    /// Tries to convert the specified string representation of a logical value to
    /// its type T equivalent. A return value indicates whether the conversion
    /// succeeded or failed.
    /// </summary>
    /// <param name="value">A string containing the value to try and convert.</param>
    /// <param name="result">If the conversion was successful, the converted value of type T.</param>
    /// <returns>If value was converted successfully, true; otherwise false.</returns>
    public static bool TryParse(string value, out T result) {
        return tryParseCached(value, out result);
    }

    /// <summary>
    /// Tries to convert the specified string representation of a logical value to
    /// its type T equivalent. A return value indicates whether the conversion
    /// succeeded or failed.
    /// </summary>
    /// <param name="value">A string containing the value to try and convert.</param>
    /// <returns>If value was converted successfully, true; otherwise false.</returns>
    public static bool TryParse(string value) {
        T throwaway;
        return TryParse(value, out throwaway);
    }
}

这样叫它:

string input = "987";
decimal myDecimal;

bool myIntSuccess = TryParseHelper<int>.TryParse(input);
bool myDecimalSuccess = TryParseHelper<decimal>.TryParse(input, out myDecimal);

其他回答

你不能在一般类型上这样做。

您可以创建一个接口ITryParsable,并将其用于实现该接口的自定义类型。

我猜你打算将它用于基本类型,如int和DateTime。您不能更改这些类型来实现新的接口。

使用try/catch进行流控制是一种糟糕的策略。抛出异常会导致运行时在处理异常时性能滞后。相反,在转换之前验证数据。

var attemptedValue = "asdfasdsd";
var type = typeof(int);
var converter = TypeDescriptor.GetConverter(type);
if (converter != null &&  converter.IsValid(attemptedValue))
    return converter.ConvertFromString(attemptedValue);
else
    return Activator.CreateInstance(type);

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;
        }
    }

受到Charlie Brown在这里发布的解决方案的启发,我使用反射创建了一个通用的TryParse,可以选择输出解析后的值:

/// <summary>
/// Tries to convert the specified string representation of a logical value to
/// its type T equivalent. A return value indicates whether the conversion
/// succeeded or failed.
/// </summary>
/// <typeparam name="T">The type to try and convert to.</typeparam>
/// <param name="value">A string containing the value to try and convert.</param>
/// <param name="result">If the conversion was successful, the converted value of type T.</param>
/// <returns>If value was converted successfully, true; otherwise false.</returns>
public static bool TryParse<T>(string value, out T result) where T : struct {
    var tryParseMethod = typeof(T).GetMethod("TryParse", BindingFlags.Static | BindingFlags.Public, null, new [] { typeof(string), typeof(T).MakeByRefType() }, null);
    var parameters = new object[] { value, null };

    var retVal = (bool)tryParseMethod.Invoke(null, parameters);

    result = (T)parameters[1];
    return retVal;
}

/// <summary>
/// Tries to convert the specified string representation of a logical value to
/// its type T equivalent. A return value indicates whether the conversion
/// succeeded or failed.
/// </summary>
/// <typeparam name="T">The type to try and convert to.</typeparam>
/// <param name="value">A string containing the value to try and convert.</param>
/// <returns>If value was converted successfully, true; otherwise false.</returns>
public static bool TryParse<T>(string value) where T : struct {
    T throwaway;
    var retVal = TryParse(value, out throwaway);
    return retVal;
}

它可以这样被称为:

string input = "123";
decimal myDecimal;

bool myIntSuccess = TryParse<int>(input);
bool myDecimalSuccess = TryParse<decimal>(input, out myDecimal);

更新: 也感谢我非常喜欢的YotaXP的解决方案,我创建了一个不使用扩展方法但仍然有一个单例的版本,最大限度地减少了反射的需要:

/// <summary>
/// Provides some extra parsing functionality for value types.
/// </summary>
/// <typeparam name="T">The value type T to operate on.</typeparam>
public static class TryParseHelper<T> where T : struct {
    private delegate bool TryParseFunc(string str, out T result);

    private static TryParseFunc tryParseFuncCached;

    private static TryParseFunc tryParseCached {
        get {
            return tryParseFuncCached ?? (tryParseFuncCached = Delegate.CreateDelegate(typeof(TryParseFunc), typeof(T), "TryParse") as TryParseFunc);
        }
    }

    /// <summary>
    /// Tries to convert the specified string representation of a logical value to
    /// its type T equivalent. A return value indicates whether the conversion
    /// succeeded or failed.
    /// </summary>
    /// <param name="value">A string containing the value to try and convert.</param>
    /// <param name="result">If the conversion was successful, the converted value of type T.</param>
    /// <returns>If value was converted successfully, true; otherwise false.</returns>
    public static bool TryParse(string value, out T result) {
        return tryParseCached(value, out result);
    }

    /// <summary>
    /// Tries to convert the specified string representation of a logical value to
    /// its type T equivalent. A return value indicates whether the conversion
    /// succeeded or failed.
    /// </summary>
    /// <param name="value">A string containing the value to try and convert.</param>
    /// <returns>If value was converted successfully, true; otherwise false.</returns>
    public static bool TryParse(string value) {
        T throwaway;
        return TryParse(value, out throwaway);
    }
}

这样叫它:

string input = "987";
decimal myDecimal;

bool myIntSuccess = TryParseHelper<int>.TryParse(input);
bool myDecimalSuccess = TryParseHelper<decimal>.TryParse(input, out myDecimal);

这是一个“一般约束”的问题。因为你没有一个特定的界面,那么你就会陷入困境,除非你遵循前面的建议。

有关这方面的文档,请查看以下链接:

http://msdn.microsoft.com/en-us/library/ms379564 (VS.80) . aspx

它向您展示了如何使用这些约束,并应该为您提供更多的线索。