如何将可空的DateTime dt2转换为格式化的字符串?
DateTime dt = DateTime.Now;
Console.WriteLine(dt.ToString("yyyy-MM-dd hh:mm:ss")); //works
DateTime? dt2 = DateTime.Now;
Console.WriteLine(dt2.ToString("yyyy-MM-dd hh:mm:ss")); //gives following error:
ToString方法没有重载 一个参数
当前回答
这个问题的问题在于，当可为空的datetime没有值时，您没有指定所需的输出。下面的代码将输出DateTime。MinValue在这种情况下，与当前接受的答案不同，将不会抛出异常。
dt2.GetValueOrDefault().ToString(format);
其他回答
iformatable还包括一个可以使用的格式提供程序，它允许IFormatProvider的两种格式在dotnet 4.0中都为空
/// <summary>
/// Extentionclass for a nullable structs
/// </summary>
public static class NullableStructExtensions {
/// <summary>
/// Formats a nullable struct
/// </summary>
/// <param name="source"></param>
/// <param name="format">The format string
/// If <c>null</c> use the default format defined for the type of the IFormattable implementation.</param>
/// <param name="provider">The format provider
/// If <c>null</c> the default provider is used</param>
/// <param name="defaultValue">The string to show when the source is <c>null</c>.
/// If <c>null</c> an empty string is returned</param>
/// <returns>The formatted string or the default value if the source is <c>null</c></returns>
public static string ToString<T>(this T? source, string format = null,
IFormatProvider provider = null,
string defaultValue = null)
where T : struct, IFormattable {
return source.HasValue
? source.Value.ToString(format, provider)
: (String.IsNullOrEmpty(defaultValue) ? String.Empty : defaultValue);
}
}
与命名参数一起使用，你可以做到:
dt2。ToString (defaultValue: n / a”);
在旧版本的dotnet中，你会遇到很多重载
/// <summary>
/// Extentionclass for a nullable structs
/// </summary>
public static class NullableStructExtensions {
/// <summary>
/// Formats a nullable struct
/// </summary>
/// <param name="source"></param>
/// <param name="format">The format string
/// If <c>null</c> use the default format defined for the type of the IFormattable implementation.</param>
/// <param name="provider">The format provider
/// If <c>null</c> the default provider is used</param>
/// <param name="defaultValue">The string to show when the source is <c>null</c>.
/// If <c>null</c> an empty string is returned</param>
/// <returns>The formatted string or the default value if the source is <c>null</c></returns>
public static string ToString<T>(this T? source, string format,
IFormatProvider provider, string defaultValue)
where T : struct, IFormattable {
return source.HasValue
? source.Value.ToString(format, provider)
: (String.IsNullOrEmpty(defaultValue) ? String.Empty : defaultValue);
}
/// <summary>
/// Formats a nullable struct
/// </summary>
/// <param name="source"></param>
/// <param name="format">The format string
/// If <c>null</c> use the default format defined for the type of the IFormattable implementation.</param>
/// <param name="defaultValue">The string to show when the source is null. If <c>null</c> an empty string is returned</param>
/// <returns>The formatted string or the default value if the source is <c>null</c></returns>
public static string ToString<T>(this T? source, string format, string defaultValue)
where T : struct, IFormattable {
return ToString(source, format, null, defaultValue);
}
/// <summary>
/// Formats a nullable struct
/// </summary>
/// <param name="source"></param>
/// <param name="format">The format string
/// If <c>null</c> use the default format defined for the type of the IFormattable implementation.</param>
/// <param name="provider">The format provider (if <c>null</c> the default provider is used)</param>
/// <returns>The formatted string or an empty string if the source is <c>null</c></returns>
public static string ToString<T>(this T? source, string format, IFormatProvider provider)
where T : struct, IFormattable {
return ToString(source, format, provider, null);
}
/// <summary>
/// Formats a nullable struct or returns an empty string
/// </summary>
/// <param name="source"></param>
/// <param name="format">The format string
/// If <c>null</c> use the default format defined for the type of the IFormattable implementation.</param>
/// <returns>The formatted string or an empty string if the source is null</returns>
public static string ToString<T>(this T? source, string format)
where T : struct, IFormattable {
return ToString(source, format, null, null);
}
/// <summary>
/// Formats a nullable struct
/// </summary>
/// <param name="source"></param>
/// <param name="provider">The format provider (if <c>null</c> the default provider is used)</param>
/// <param name="defaultValue">The string to show when the source is <c>null</c>. If <c>null</c> an empty string is returned</param>
/// <returns>The formatted string or the default value if the source is <c>null</c></returns>
public static string ToString<T>(this T? source, IFormatProvider provider, string defaultValue)
where T : struct, IFormattable {
return ToString(source, null, provider, defaultValue);
}
/// <summary>
/// Formats a nullable struct or returns an empty string
/// </summary>
/// <param name="source"></param>
/// <param name="provider">The format provider (if <c>null</c> the default provider is used)</param>
/// <returns>The formatted string or an empty string if the source is <c>null</c></returns>
public static string ToString<T>(this T? source, IFormatProvider provider)
where T : struct, IFormattable {
return ToString(source, null, provider, null);
}
/// <summary>
/// Formats a nullable struct or returns an empty string
/// </summary>
/// <param name="source"></param>
/// <returns>The formatted string or an empty string if the source is <c>null</c></returns>
public static string ToString<T>(this T? source)
where T : struct, IFormattable {
return ToString(source, null, null, null);
}
}
您可以使用dt2. value . tostring ("format")，但当然这需要dt2 != null，这首先就否定了可空类型的使用。
这里有几个解决方案，但最大的问题是:您希望如何格式化空日期?
我喜欢这个选项:
Console.WriteLine(dt2?.ToString("yyyy-MM-dd hh:mm:ss") ?? "n/a");
Console.WriteLine(dt2 != null ? dt2.Value.ToString("yyyy-MM-dd hh:mm:ss") : "n/a");
编辑:如其他注释中所述，检查是否有一个非空值。
更新:如评论中推荐，扩展方法:
public static string ToString(this DateTime? dt, string format)
=> dt == null ? "n/a" : ((DateTime)dt).ToString(format);
从c# 6开始，您可以使用空条件操作符来进一步简化代码。如果DateTime?是零。
dt2?.ToString("yyyy-MM-dd hh:mm:ss")
你们把事情搞得太复杂了。重要的是，停止使用ToString，开始使用字符串格式，比如string。支持字符串格式的格式或方法，如Console.WriteLine。下面是这个问题的最佳解决方案。这也是最安全的。
更新:
我用当前c#编译器的最新方法更新了示例。条件操作符和字符串插值
DateTime? dt1 = DateTime.Now;
DateTime? dt2 = null;
Console.WriteLine("'{0:yyyy-MM-dd hh:mm:ss}'", dt1);
Console.WriteLine("'{0:yyyy-MM-dd hh:mm:ss}'", dt2);
// New C# 6 conditional operators (makes using .ToString safer if you must use it)
// https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-conditional-operators
Console.WriteLine(dt1?.ToString("yyyy-MM-dd hh:mm:ss"));
Console.WriteLine(dt2?.ToString("yyyy-MM-dd hh:mm:ss"));
// New C# 6 string interpolation
// https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated
Console.WriteLine($"'{dt1:yyyy-MM-dd hh:mm:ss}'");
Console.WriteLine($"'{dt2:yyyy-MM-dd hh:mm:ss}'");
输出:(我在里面放了单引号，所以你可以看到它返回为空字符串时为null)
'2019-04-09 08:01:39'
''
2019-04-09 08:01:39
'2019-04-09 08:01:39'
''
