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Cast string to decimal? (nullable decimal) by extension method

I want to cast a string to a decimal?. With reference to a previous question here
one of the answers gave a object extension to convert objects like so
public static class ObjectExtensions
{
public static Nullable<T> ToNullable<T>(this object input)
where T : struct
{
if (input == null)
return null;
if (input is Nullable<T> || input is T)
return (Nullable<T>)input;
throw new InvalidCastException();
}
}
usage :
object value = 123.45m;
decimal? dec = value.ToNullable<decimal>();
The above does not work with string however, is it possible to define a companion to the above method to specifically handle strings?
specifically like what I would like to be able to do is :-
object v1 = 123.45m;
decimal? d1 = v1.ToNullable<decimal>();
object v2 = "123.45";
decimal? d2 = v2.ToNullable<decimal>();

A simple extension method converts string to decimal. For not null strings of course
public static class StringExtensions
{
public static decimal? ToNullableDecimal(this string s)
{
decimal value;
if (!Decimal.TryParse(s, out value)
return null;
return value;
}
}

Given your extension function, why not something like :
public static class ObjectExtensions
{
public static Nullable<T> ToNullable<T>(this object input)
where T : struct
{
if (input == null)
return null;
if (input is Nullable<T> || input is T)
return (Nullable<T>)input;
else if (input is string)
return (T)Convert.ChangeType(input, typeof(T));
throw new InvalidCastException();
}
}
It will work for numeric types (supported by ChangeType), and even Dates and so.
Of course, "input" must be convertible to desired type (beware of culture-specific constraints).
To improve this, you could pass to "ChangeType" (at third parameter) the Culture you want to work with, for example return ((T)Convert.ChangeType(input, typeof(T), new CultureInfo("en-US")));
You can also add stuff in this method to handle exceptions, etc

You could do something like this:
string nds = null;
decimal? d = (nds != null) ? Convert.ToDecimal(nds) : default(decimal?);

Here is what is work for me
1- Create following method inside a class (ex ValidationHelper)
public static decimal? Todecimal(string s,decimal defValue=0)
{
if (s.Trim()!="")
{
return Convert.ToDecimal(s);
}
else
{
return defValue;
}
}
and then you can use it anywhere in your app like this
prod.Qty = ValidationHelper.Todecimal(txtQty.Text,1);//ex: assuming the default value for Qty is 1.
Hope this will help someone. Thanks

C# enum contains value

I have an enum
enum myEnum2 { ab, st, top, under, below}
I would like to write a function to test if a given value is included in myEnum
something like that:
private bool EnumContainValue(Enum myEnum, string myValue)
{
return Enum.GetValues(typeof(myEnum))
.ToString().ToUpper().Contains(myValue.ToUpper());
}
But it doesn't work because myEnum parameter is not recognized.

Why not use
Enum.IsDefined(typeof(myEnum), value);
BTW it's nice to create generic Enum<T> class, which wraps around calls to Enum (actually I wonder why something like this was not added to Framework 2.0 or later):
public static class Enum<T>
{
public static bool IsDefined(string name)
{
return Enum.IsDefined(typeof(T), name);
}
public static bool IsDefined(T value)
{
return Enum.IsDefined(typeof(T), value);
}
public static IEnumerable<T> GetValues()
{
return Enum.GetValues(typeof(T)).Cast<T>();
}
// etc
}
This allows to avoid all this typeof stuff and use strongly-typed values:
Enum<StringSplitOptions>.IsDefined("None")

No need to write your own:
// Summary:
// Returns an indication whether a constant with a specified value exists in
// a specified enumeration.
//
// Parameters:
// enumType:
// An enumeration type.
//
// value:
// The value or name of a constant in enumType.
//
// Returns:
// true if a constant in enumType has a value equal to value; otherwise, false.
public static bool IsDefined(Type enumType, object value);
Example:
if (System.Enum.IsDefined(MyEnumType, MyValue))
{
// Do something
}

just use this method
Enum.IsDefined Method - Returns an indication whether a constant with a specified value exists in a specified enumeration
Example
enum myEnum2 { ab, st, top, under, below};
myEnum2 value = myEnum2.ab;
Console.WriteLine("{0:D} Exists: {1}",
value, myEnum2.IsDefined(typeof(myEnum2), value));

What you're doing with ToString() in this case is to:
Enum.GetValues(typeof(myEnum)).ToString()... instead you should write:
Enum.GetValues(typeof(myEnum).ToString()...
The difference is in the parentheses...

Also can use this:
enum myEnum2 { ab, st, top, under, below }
static void Main(string[] args)
{
myEnum2 r;
string name = "ab";
bool result = Enum.TryParse(name, out r);
}
The result will contain whether the value is contained in enum or not.

public static T ConvertToEnum<T>(this string value)
{
if (typeof(T).BaseType != typeof(Enum))
{
throw new InvalidCastException("The specified object is not an enum.");
}
if (Enum.IsDefined(typeof(T), value.ToUpper()) == false)
{
throw new InvalidCastException("The parameter value doesn't exist in the specified enum.");
}
return (T)Enum.Parse(typeof(T), value.ToUpper());
}

If your question is like "I have an enum type, enum MyEnum { OneEnumMember, OtherEnumMember }, and I'd like to have a function which tells whether this enum type contains a member with a specific name, then what you're looking for is the System.Enum.IsDefined method:
Enum.IsDefined(typeof(MyEnum), MyEnum.OneEnumMember); //returns true
Enum.IsDefined(typeof(MyEnum), "OtherEnumMember"); //returns true
Enum.IsDefined(typeof(MyEnum), "SomethingDifferent"); //returns false
If your question is like "I have an instance of an enum type, which has Flags attribute, and I'd like to have a function which tells whether this instance contains a specific enum value, then the function looks something like this:
public static bool ContainsValue<TEnum>(this TEnum e, TEnum val) where Enum: struct, IComparable, IFormattable, IConvertible
{
if (!e.GetType().IsEnum)
throw new ArgumentException("The type TEnum must be an enum type.", nameof(TEnum));
dynamic val1 = e, val2 = val;
return (val1 | val2) == val1;
}
Hope I could help.

Use the correct name of the enum (myEnum2).
Also, if you're testing against a string value you may want to use GetNames instead of GetValues.

just cast the enum as:
string something = (string)myEnum;
and now comparison is easy as you like

I think that you go wrong when using ToString().
Try making a Linq query
private bool EnumContainValue(Enum myEnum, string myValue)
{
var query = from enumVal in Enum.GetNames(typeof(GM)).ToList()
where enumVal == myValue
select enumVal;
return query.Count() == 1;
}

Elegant TryParse

I feel that every time I use TryParse that it results in somewhat ugly code. Mainly I am using it this way:
int value;
if (!int.TryParse(someStringValue, out value))
{
value = 0;
}
Is there some more elegant solution for parsing all basic data types, to be specific is there a way to do fail safe parsing in one line? By fail safe I assume setting default value if parsing fails without exception.
By the way, this is for cases where I must do some action even if parsing fails, just using the default value.

This is valid and you may prefer it if you have a liking for single-liners:
int i = int.TryParse(s, out i) ? i : 42;
This sets the value of i to 42 if it cannot parse the string s, otherwise it sets i = i.

how about a direct extension method?
public static class Extensions
{
public static int? TryParse(this string Source)
{
int result;
if (int.TryParse(Source, out result))
return result;
else
return null;
}
}
or with the new c# syntax in a single line:
public static int? TryParse(this string Source) => int.TryParse(Source, out int result) ? result : (int?)null;
usage:
v = "234".TryParse() ?? 0

You can write your own methods for a solution that suits you better. I stumbled upon the Maybe class that wraps the TryParse methods a while ago.
int? value = Maybe.ToInt("123");
if (value == null)
{
// not a number
}
else
{
// use value.Value
}
or specify the default value in-line:
int value = Maybe.ToInt("123") ?? 0;
Observe the distinction between Nullable<int>/int? and int.
See http://www.kodefuguru.com/post/2010/06/24/TryParse-vs-Convert.aspx for more info

There is a nice little feature in C# 6 C# 7, Declaration expressions, so in C# 7 instead of:
int x;
if (int.TryParse("123", out x))
{
DoSomethingWithX(x);
}
we can use:
if (int.TryParse("123", out int x))
{
DoSomethingWithX(x);
}
Nice enough for me :)

using C# 7 in single line
int.TryParse(s, out var i) ? i : (int?)null;
Example Method:
public static int? TryParseSafe(string s)
{
return int.TryParse(s, out var i) ? i : (int?)null;
}

In your particular example, you can do this:
int value;
int.TryParse(someStringValue, out value);
...because Int32.TryParse() is documented as setting value=0 if it fails the parse.

You could use TypeDescriptor instead:
public T Convert<T>(string input, T defaultVal)
{
var converter = System.ComponentModel.TypeDescriptor.GetConverter(typeof(T));
if(converter != null)
{
return (T)converter.ConvertFromString(input);
}
return defaultVal;
}
public T Convert<T>(string input)
{
return Convert(input, default(T));
}
You could constrain T to struct and use Nullable also (as per #skarmats answer).

This is one of the nice surprises for C# developers who try F#.
The TryParse method returns a tuple containing both the bool and the value.

How to write generic extension methods?

I am developing a generic wrapper around TryParse, as follows:
public delegate bool ParseDelegate<T>(string s, out T result);
public static T? ParseOrNull<T>(this string value, ParseDelegate<T> parse) where T : struct
{
T result;
var parsed = parse(value, out result);
return parsed ? result : (T?)null;
}
[Test]
public void ParsesValidInt()
{
Assert.AreEqual(1234, "1234".ParseOrNull<int>(int.TryParse));
}
[Test]
public void ParsesValidDecimal()
{
Assert.AreEqual(12.34M, "12.34".ParseOrNull<decimal>(decimal.TryParse));
}
This is kinda repetitive. Is there a way to avoid mentioning int.TryParse at all, so that my calls look as follows:
"1234".ParseOrNull<int>()

Is there a way to avoid mentioning int.TryParse at all, so that my calls look as follows:
Not directly, as TryParse isn't part of a shared interface. If there were a shared interface to these value types, this would be possible via a constraint.
Personally, I would not suggest using extension methods for this. I would rather write this as something more like:
public static class Parse
{
public delegate bool ParseDelegate<T>(string s, out T result);
public static T? FromString<T>(string value, ParseDelegate<T> parse) where T : struct
{
T result;
var parsed = parse(value, out result);
return parsed ? result : (T?)null;
}
public static int? ToNullableInt32(string value)
{
return FromString<int>(value, int.TryParse);
}
public static double? ToNullableDouble(string value)
{
return FromString<double>(value, double.TryParse);
}
}
This adds a bit of overhead up front, but allows you to write these very cleanly, ie:
int? first = Parse.FromString<int>("1234", int.TryParse);
int? second = Parse.ToNullableInt32("1234");
double? third = Parse.ToNullableDouble("1234");
I see little value in putting an extension method, especially on something like string (which is used everywhere), as it "pollutes" the compilation of string itself. You'll see this everywhere you use strings - basically, any time you use this namespace, you'll end up having these parse methods in your intellisense, etc. In addition, this seems more like a "utility" than something that should appear as built-in functionality of string itself, which is why I personally prefer a separate class for it.

In short no but you can add a new helper method:
public static int? ParseInt(this string value)
{
return value.ParseOrNull<int>(int.TryParse);
}
and then:
"1234".ParseInt();

Look at how Microsoft deals with several types . They provides one method for each type. Enumerable.Sum Method is a good example. If you want to simplify the calling code, you should provide the overloads for each types :
public static int? ParseOrNull<int>(this string value)
{
int result;
var parsed = int.TryParse(value, out result);
return parsed ? result : (T?)null;
}
public static long? ParseOrNull<long>(this string value)
{
long result;
var parsed = long.TryParse(value, out result);
return parsed ? result : (T?)null;
}
// same for ulong, long, uint, ushort, short, byte,
// bool, float, double, decimal. Do I forget one ?
I think it's more important to simplify calls than the method itself. In fact, there is not a huge number of types to deal with.

Yes, you can use Convert.ChangeType
public static T? ParseOrNull<T>(this string value) where T : struct, IConvertible
{
try
{
return (T)Convert.ChangeType(value, typeof(T));
}
catch (FormatException ex)
{
return null;
}
}
It wont have as good performance (use of try catch) as TryParse, but should work for all IConvertible types

The answer is a big YES. You're trying to exploit the existence of the static T.TryParse(string, out T) function on the types you're converting to, and we can do that pretty easily with a little reflection.
public static T? ParseOrNull<T>(this string str)
where T: struct, IConvertible
{
// find the TryParse method.
var parseMethod = typeof(T).GetMethod("TryParse",
// We want the public static one
BindingFlags.Public | BindingFlags.Static,
Type.DefaultBinder,
// where the arguments are (string, out T)
new[] { typeof(string), typeof(T).MakeByRefType() },
null);
if (parseMethod == null)
// You need to know this so you can parse manually
throw new InvalidOperationException(
string.Format("{0} doesn't have a TryParse(..) function!",
typeof(T).FullName));
// create the parameter list for the function call
var args = new object[] { str, default(T) };
// and then call the function.
if ( (bool)parseMethod.Invoke(null, args))
return (T?)args[1]; // if it returned true
// if it returned false
return null;
}
This is the original answer I provided, based on the idea that you need two different parse methods: One for value types and another for reference types.
public delegate bool ParseDelegate<T>(string s, out T result);
public static T? ParseOrNull<T>(this string str, ParseDelegate<T> Parse)
where T: struct
{
T result;
if (!Parse(str, out result))
return null;
return result;
}
public static T ParseOrNull<T>(this string str, ParseDelegate<T> Parse)
where T : class
{
T result;
if (!Parse(str, out result))
return null;
return result;
}

public static T? ParseOrNull<T>(this string value)
where T : struct
{
T result = default(T);
object[] parameters = new object[] { value, result };
foreach (System.Reflection.MethodInfo method in
typeof(T).GetMethods()
.Where(method => method.Name == "TryParse")
.Where(method => method.GetParameters().Length == 2) //as opposed to the 4 argument version
.Take(1) //shouldn't be needed, but just in case
)
{
method.Invoke(null, parameters);
}
return (T)parameters[1];
}
As Reed mentions, I'd rather not use an extension method of string. I'd just use Parser.Parse(string value). Easy fix though, just remove the 'this' and voila.

C# Generics and Type Checking

I have a method that uses an IList<T> as a parameter. I need to check what the type of that T object is and do something based on it. I was trying to use the T value, but the compiler does not not allow it. My solution is the following:
private static string BuildClause<T>(IList<T> clause)
{
if (clause.Count > 0)
{
if (clause[0] is int || clause[0] is decimal)
{
//do something
}
else if (clause[0] is String)
{
//do something else
}
else if (...) //etc for all the types
else
{
throw new ApplicationException("Invalid type");
}
}
}
There has to be a better way to do this. Is there some way I can check the type of T that is passed in and then use a switch statement?

You could use overloads:
public static string BuildClause(List<string> l){...}
public static string BuildClause(List<int> l){...}
public static string BuildClause<T>(List<T> l){...}
Or you could inspect the type of the generic parameter:
Type listType = typeof(T);
if(listType == typeof(int)){...}

You can use typeof(T).
private static string BuildClause<T>(IList<T> clause)
{
Type itemType = typeof(T);
if(itemType == typeof(int) || itemType == typeof(decimal))
...
}

And, because C# has evolved, you can (now) use pattern matching.
private static string BuildClause<T>(IList<T> clause)
{
if (clause.Count > 0)
{
switch (clause[0])
{
case int x: // do something with x, which is an int here...
case decimal x: // do something with x, which is a decimal here...
case string x: // do something with x, which is a string here...
...
default: throw new Exception("Invalid type");
}
}
}
And again with switch expressions in C# 8.0, the syntax gets even more succinct.
private static string BuildClause<T>(IList<T> clause)
{
if (clause.Count > 0)
{
return clause[0] switch
{
int x => "some string related to this int",
decimal x => "some string related to this decimal",
string x => x,
...,
_ => throw new Exception("Invalid type")
}
}
}

I hope you find this helpful:
typeof(IList<T>).IsGenericType == true
typeof(IList<T>).GetGenericTypeDefinition() == typeof(IList<>)
typeof(IList<int>).GetGenericArguments()[0] == typeof(int)
https://dotnetfiddle.net/5qUZnt

By default know there is not a great way. Awhile back I got frustrated with this and wrote a little utility class that helped out a bit and made the syntax a bit cleaner. Essentially it turns the code into
TypeSwitcher.Do(clause[0],
TypeSwitch.Case<int>(x => ...), // x is an int
TypeSwitch.Case<decimal>(d => ...), // d is a decimal
TypeSwitch.Case<string>(s => ...)); // s is a string
Full blog post and details on the implementation are available here
http://blogs.msdn.com/jaredpar/archive/2008/05/16/switching-on-types.aspx

The typeof operator...
typeof(T)
... won't work with the c# switch statement. But how about this? The following post contains a static class...
Is there a better alternative than this to 'switch on type'?
...that will let you write code like this:
TypeSwitch.Do(
sender,
TypeSwitch.Case<Button>(() => textBox1.Text = "Hit a Button"),
TypeSwitch.Case<CheckBox>(x => textBox1.Text = "Checkbox is " + x.Checked),
TypeSwitch.Default(() => textBox1.Text = "Not sure what is hovered over"));

There is no way to use the switch statement for what you want it to do. The switch statement must be supplied with integral types, which does not include complex types such as a "Type" object, or any other object type for that matter.

For everyone that says checking types and doing something based on the type is not a great idea for generics I sort of agree but I think there could be some circumstances where this perfectly makes sense.
For example if you have a class that say is implemented like so (Note: I am not showing everything that this code does for simplicity and have simply cut and pasted into here so it may not build or work as intended like the entire code does but it gets the point across. Also, Unit is an enum):
public class FoodCount<TValue> : BaseFoodCount
{
public TValue Value { get; set; }
public override string ToString()
{
if (Value is decimal)
{
// Code not cleaned up yet
// Some code and values defined in base class
mstrValue = Value.ToString();
decimal mdecValue;
decimal.TryParse(mstrValue, out mdecValue);
mstrValue = decimal.Round(mdecValue).ToString();
mstrValue = mstrValue + mstrUnitOfMeasurement;
return mstrValue;
}
else
{
// Simply return a string
string str = Value.ToString() + mstrUnitOfMeasurement;
return str;
}
}
}
...
public class SaturatedFat : FoodCountWithDailyValue<decimal>
{
public SaturatedFat()
{
mUnit = Unit.g;
}
}
public class Fiber : FoodCount<int>
{
public Fiber()
{
mUnit = Unit.g;
}
}
public void DoSomething()
{
nutritionFields.SaturatedFat oSatFat = new nutritionFields.SaturatedFat();
string mstrValueToDisplayPreFormatted= oSatFat.ToString();
}
So in summary, I think there are valid reasons why you might want to check to see what type the generic is, in order to do something special.

Your construction completely defeats the purpose of a generic method. It's ugly on purpose because there must be a better way to achieve what you're trying to accomplish, although you haven't given us quite enough information to figure out what that is.

You can do typeOf(T), but I would double check your method and make sure your not violating single responsability here. This would be a code smell, and that's not to say it shouldn't be done but that you should be cautious.
The point of generics is being able to build type-agnostic algorthims were you don't care what the type is or as long as it fits within a certain set of criteria. Your implementation isn't very generic.

How about this :
// Checks to see if the value passed is valid.
if (!TypeDescriptor.GetConverter(typeof(T)).IsValid(value))
{
throw new ArgumentException();
}

My two cents:
In case you happen to have a generic method that returns a generic value but doesn't have generic parameters, you can use default(T) + (T)(object) cast, together with C# 8 pattern matching/type checks (as indicated in the other recent answers).
Example:
private static T Parse<T>(string str)
{
return default(T) switch
{
short => (T)(object)short.Parse(str),
ushort => (T)(object)ushort.Parse(str),
int => (T)(object)int.Parse(str),
uint => (T)(object)uint.Parse(str),
long => (T)(object)long.Parse(str),
ulong => (T)(object)ulong.Parse(str),
_ => throw new ArgumentException()
};
}