c# decimal? - Convert string to nullable type(int, double, etc…)



object how (14)

I am attempting to do some data conversion. Unfortunately, much of the data is in strings, where it should be int's or double, etc...

So what I've got is something like:

double? amount = Convert.ToDouble(strAmount);

The problem with this approach is if strAmount is empty, if it's empty I want it to amount to be null, so when I add it into the database the column will be null. So I ended up writing this:

double? amount = null;
if(strAmount.Trim().Length>0)
{
    amount = Convert.ToDouble(strAmount);
}

Now this works fine, but I now have five lines of code instead of one. This makes things a little more difficult to read, especially when I have a large amount of columns to convert.

I thought I'd use an extension to the string class and generic's to pass in the type, this is because it could be a double, or an int, or a long. So I tried this:

    public static class GenericExtension
    {
        public static Nullable<T> ConvertToNullable<T>(this string s, T type) where T: struct
        {
            if (s.Trim().Length > 0)
            {
                return (Nullable<T>)s;
            }
            return null;
        }
    }

But I get the error: Cannot convert type 'string' to 'T?'

Is there a way around this? I am not very familiar with creating methods using generics.


Answers

I like Joel's answer, but I've modified it slightly as I'm not a fan of eating exceptions.

    /// <summary>
    /// Converts a string to the specified nullable type.
    /// </summary>
    /// <typeparam name="T">The type to convert to</typeparam>
    /// <param name="s">The string to convert</param>
    /// <returns>The nullable output</returns>
    public static T? ToNullable<T>(this string s) where T : struct
    {
        if (string.IsNullOrWhiteSpace(s))
            return null;

        TypeConverter conv = TypeDescriptor.GetConverter(typeof (T));
        return (T) conv.ConvertFrom(s);
    }

    /// <summary>
    /// Attempts to convert a string to the specified nullable primative.
    /// </summary>
    /// <typeparam name="T">The primitive type to convert to</typeparam>
    /// <param name="data">The string to convert</param>
    /// <param name="output">The nullable output</param>
    /// <returns>
    /// True if conversion is successfull, false otherwise.  Null and whitespace will
    /// be converted to null and return true.
    /// </returns>
    public static bool TryParseNullable<T>(this string data, out T? output) where T : struct
    {
        try
        {
            output = data.ToNullable<T>();
            return true;
        }
        catch
        {
            output = null;
            return false;
        }
    }

Another variation. This one

  • Does not swallow exceptions
  • Throws a NotSupportedException if the type can not be converted from string. For instance, a custom struct without a type converter.
  • Otherwise returns a (T?)null if the string fails to parse. No need to check for null or whitespace.
using System.ComponentModel;

public static Nullable<T> ToNullable<T>(this string s) where T : struct
{
    var ret = new Nullable<T>();
    var conv = TypeDescriptor.GetConverter(typeof(T));

    if (!conv.CanConvertFrom(typeof(string)))
    {
        throw new NotSupportedException();
    }

    if (conv.IsValid(s))
    {
        ret = (T)conv.ConvertFrom(s);
    }

    return ret;
}

Let's add one more similar solution to the stack. This one also parses enums, and it looks nice. Very safe.

/// <summary>
    /// <para>More convenient than using T.TryParse(string, out T). 
    /// Works with primitive types, structs, and enums.
    /// Tries to parse the string to an instance of the type specified.
    /// If the input cannot be parsed, null will be returned.
    /// </para>
    /// <para>
    /// If the value of the caller is null, null will be returned.
    /// So if you have "string s = null;" and then you try "s.ToNullable...",
    /// null will be returned. No null exception will be thrown. 
    /// </para>
    /// <author>Contributed by Taylor Love (Pangamma)</author>
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="p_self"></param>
    /// <returns></returns>
    public static T? ToNullable<T>(this string p_self) where T : struct
    {
        if (!string.IsNullOrEmpty(p_self))
        {
            var converter = System.ComponentModel.TypeDescriptor.GetConverter(typeof(T));
            if (converter.IsValid(p_self)) return (T)converter.ConvertFromString(p_self);
            if (typeof(T).IsEnum) { T t; if (Enum.TryParse<T>(p_self, out t)) return t;}
        }

        return null;
    }

https://github.com/Pangamma/PangammaUtilities-CSharp/blob/master/PangammaUtilities/Extensions/ToNullableStringExtension.cs


Another thing to keep in mind is that the string itself might be null.

public static Nullable<T> ToNullable<T>(this string s) where T: struct
{
    Nullable<T> result = new Nullable<T>();
    try
    {
        if (!string.IsNullOrEmpty(s) && s.Trim().Length > 0)
        {
            TypeConverter conv = TypeDescriptor.GetConverter(typeof(T));
            result = (T)conv.ConvertFrom(s);
        }
    }
    catch { } 
    return result;
}

You can use the following with objects, unfortunately this does not work with strings though.

double? amount = (double?)someObject;

I use it for wrapping a session variable in a property (on a base page).. so my actual usage is (in my base page):

public int? OrganisationID
{
    get { return (int?)Session[Constants.Session_Key_OrganisationID]; }
    set { Session[Constants.Session_Key_OrganisationID] = value; }
}

I'm able to check for null in page logic:

if (base.OrganisationID == null)
    // do stuff

My example for anonimous types:

private object ConvertNullable(object value, Type nullableType)
{
    Type resultType = typeof(Nullable<>).MakeGenericType(nullableType.GetGenericArguments());
    return Activator.CreateInstance(resultType, Convert.ChangeType(value, nullableType.GetGenericArguments()[0]));
}

...

Type anonimousType = typeof(Nullable<int>);
object nullableInt1 = ConvertNullable("5", anonimousType);
// or evident Type
Nullable<int> nullableInt2 = (Nullable<int>)ConvertNullable("5", typeof(Nullable<int>));

There is no way around this. Nullable, as well as your method, is constrained to using only value types as it's argument. String is a reference type and hence is incompatible with this declaration.


I wrote this generic type converter. It works with Nullable and standard values, converting between all convertible types - not just string. It handles all sorts of scenarios that you'd expect (default values, null values, other values, etc...)

I've been using this for about a year in dozens of production programs, so it should be pretty solid.

    public static T To<T>(this IConvertible obj)
    {
        Type t = typeof(T);

        if (t.IsGenericType
            && (t.GetGenericTypeDefinition() == typeof(Nullable<>)))
        {
            if (obj == null)
            {
                return (T)(object)null;
            }
            else
            {
                return (T)Convert.ChangeType(obj, Nullable.GetUnderlyingType(t));
            }
        }
        else
        {
            return (T)Convert.ChangeType(obj, t);
        }
    }

    public static T ToOrDefault<T>
                 (this IConvertible obj)
    {
        try
        {
            return To<T>(obj);
        }
        catch
        {
            return default(T);
        }
    }

    public static bool ToOrDefault<T>
                        (this IConvertible obj,
                         out T newObj)
    {
        try
        {
            newObj = To<T>(obj);
            return true;
        }
        catch
        {
            newObj = default(T);
            return false;
        }
    }

    public static T ToOrOther<T>
                           (this IConvertible obj,
                           T other)
    {
        try
        {
            return To<T>(obj);
        }
        catch
        {
            return other;
        }
    }

    public static bool ToOrOther<T>
                             (this IConvertible obj,
                             out T newObj,
                             T other)
    {
        try
        {
            newObj = To<T>(obj);
            return true;
        }
        catch
        {
            newObj = other;
            return false;
        }
    }

    public static T ToOrNull<T>
                          (this IConvertible obj)
                          where T : class
    {
        try
        {
            return To<T>(obj);
        }
        catch
        {
            return null;
        }
    }

    public static bool ToOrNull<T>
                      (this IConvertible obj,
                      out T newObj)
                      where T : class
    {
        try
        {
            newObj = To<T>(obj);
            return true;
        }
        catch
        {
            newObj = null;
            return false;
        }
    }

You might want to try:

TypeConverter conv = TypeDescriptor.GetConverter(typeof(int));
conv.ConvertFrom(mystring);

do your own null check and return int? if necessary. You'll also want to wrap that in a try {}


The generic answer provided by "Joel Coehoorn" is good.

But, this is another way without using those GetConverter... or try/catch blocks... (i'm not sure but this may have better performance in some cases):

public static class StrToNumberExtensions
{
    public static short ToShort(this string s, short defaultValue = 0) => short.TryParse(s, out var v) ? v : defaultValue;
    public static int ToInt(this string s, int defaultValue = 0) => int.TryParse(s, out var v) ? v : defaultValue;
    public static long ToLong(this string s, long defaultValue = 0) => long.TryParse(s, out var v) ? v : defaultValue;
    public static decimal ToDecimal(this string s, decimal defaultValue = 0) => decimal.TryParse(s, out var v) ? v : defaultValue;
    public static float ToFloat(this string s, float defaultValue = 0) => float.TryParse(s, out var v) ? v : defaultValue;
    public static double ToDouble(this string s, double defaultValue = 0) => double.TryParse(s, out var v) ? v : defaultValue;

    public static short? ToshortNullable(this string s, short? defaultValue = null) => short.TryParse(s, out var v) ? v : defaultValue;
    public static int? ToIntNullable(this string s, int? defaultValue = null) => int.TryParse(s, out var v) ? v : defaultValue;
    public static long? ToLongNullable(this string s, long? defaultValue = null) => long.TryParse(s, out var v) ? v : defaultValue;
    public static decimal? ToDecimalNullable(this string s, decimal? defaultValue = null) => decimal.TryParse(s, out var v) ? v : defaultValue;
    public static float? ToFloatNullable(this string s, float? defaultValue = null) => float.TryParse(s, out var v) ? v : defaultValue;
    public static double? ToDoubleNullable(this string s, double? defaultValue = null) => double.TryParse(s, out var v) ? v : defaultValue;
}

Usage is as following:

var x1 = "123".ToInt(); //123
var x2 = "abc".ToInt(); //0
var x3 = "abc".ToIntNullable(); // (int?)null 
int x4 = ((string)null).ToInt(-1); // -1
int x5 = "abc".ToInt(-1); // -1

var y = "19.50".ToDecimal(); //19.50

var z1 = "invalid number string".ToDoubleNullable(); // (double?)null
var z2 = "invalid number string".ToDoubleNullable(0); // (double?)0

public static class GenericExtension
{
    public static T? ConvertToNullable<T>(this String s) where T : struct 
    {
        try
        {
            return (T?)TypeDescriptor.GetConverter(typeof(T)).ConvertFrom(s);
        }
        catch (Exception)
        {
            return null;
        }
    }
}

There is a generic solution (for any type). Usability is good, but implementation should be improved: http://cleansharp.de/wordpress/2011/05/generischer-typeconverter/

This allows you to write very clean code like this:

string value = null;
int? x = value.ConvertOrDefault<int?>();

and also:

object obj = 1;  

string value = null;
int x = 5;
if (value.TryConvert(out x))
    Console.WriteLine("TryConvert example: " + x); 

bool boolean = "false".ConvertOrDefault<bool>();
bool? nullableBoolean = "".ConvertOrDefault<bool?>();
int integer = obj.ConvertOrDefault<int>();
int negativeInteger = "-12123".ConvertOrDefault<int>();
int? nullableInteger = value.ConvertOrDefault<int?>();
MyEnum enumValue = "SecondValue".ConvertOrDefault<MyEnum>();

MyObjectBase myObject = new MyObjectClassA();
MyObjectClassA myObjectClassA = myObject.ConvertOrDefault<MyObjectClassA>();

You could try using the below extension method:

public static T? GetValueOrNull<T>(this string valueAsString)
    where T : struct 
{
    if (string.IsNullOrEmpty(valueAsString))
        return null;
    return (T) Convert.ChangeType(valueAsString, typeof(T));
}

This way you can do this:

double? amount = strAmount.GetValueOrNull<double>();
int? amount = strAmount.GetValueOrNull<int>();
decimal? amount = strAmount.GetValueOrNull<decimal>();

I find that using Apache Commons IO makes my life much easier.

String source = "This is the source of my input stream";
InputStream in = org.apache.commons.io.IOUtils.toInputStream(source, "UTF-8");

You may find that the library also offer many other shortcuts to commonly done tasks that you may be able to use in your project.





c# .net generics extension-methods type-conversion