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How to convert a nullable datetime value to string.empty when datetime is null?

I'm reading back a nullable DateTime? property then assigning that value to a string property in short date format.
I can convert the date time value to a short date string and assign to the IT_Date_String property. But I'm not sure how to assign a "" value to the string if the IT_Date is null.
How can you convert a datetime? value to string.empty when datetime? is null?
This is the assignment in linq:
var status_list = query_all.ToList().Select(r => new RelStatus
{
IT_Date_String = r.IT_Date.Value.ToString("yyyy-MM-dd") != null ? r.IT_Date.Value : null
}).ToList();
And the properties in the model:
public DateTime? IT_Date { get; set; }
public string IT_Date_String { get; set; }

You're calling the IT_Date.Value.ToString(...) regardless of whether IT_Date actually has a value.
So you need to turn the expression around:
r.IT_Date.HasValue ? r.IT_Date.Value.ToString(...) : ""
This way ToString() will only be called when IT_Date has a value.
You can also implement this in the getter, as mentioned in a now-deleted comment:
public string IT_Date_String
{
get
{
return IT_Date.HasValue ? IT_Date.Value.ToString(...) : "";
}
}
This way you won't have to reimplement the logic everywhere you access this model, and as a bonus, it will only be executed when it's actually requested.
There's also no need to explicitly use String.Empty, the string "" will be interned to the same at runtime.

In C# 6 you can do this:
IT_Date_String = r.IT_Date?.ToString("yyyy-MM-dd") ?? String.Empty;
The new ? checks if the thing on the left is null, if it is, the expression evaluates to null. If not, it just continues the evaluation.
Then, ?? checks whether the result of the first expression is null, which it would be if IT_Date is null. If it is, evaluate to String.Empty.

With C# 6.0 and null propagation you can use:
IT_Date_String = r.IT_Date?.ToString("yyyy-MM-dd") ?? String.Empty

This one will work in any version of the framework:
IT_Date_String=string.Format("{0:yyyy-MM-dd}",IT_Date);

I have a method that returns string however the variable is of type nullable boolean

I get a build error in the following:
protected override string SHow()
{
return _type ? "Y" : "N";
}
_type is of type nullable bool. I get that "cannot convert from ?bool to bool.
Could you please help resolve?

A nullable type provides access to its underlying value with the Value property.
return _type.HasValue && _type.Value ? "Y" : "N";
OK, editing to return null...
const string trueValue = "True";
const string falseValue = "False";
return _type.HasValue ? (_type.Value ? trueValue : falseValue) : null;

Use
protected override string SHow()
{
if(_type ==null)
{
return "";
}
return _type.Value ? "Y" : "N";
}

If bool? has value then It is indeed true or false. Besides nullable.HasValue is required when we want to access value not when we compare value. So the following will also work:
protected override string SHow()
{
string result = null;
if(_type==true)
result = "Y"; // has value & value is true
else if (_type==false)
result="N"; // has value & value is false
return result; // retuing null if no value
}
Statement "_type==true" will never fail or give you a compile error because compiler not only check for null, it also check for true or false and return true only if _type is NOT Null and also has value true.
Equality comparison for any primitive type nullable behaves the same way so nullable.HasValue check is not necessary when you compare.

How to structure this variable

Right now the form can send me three values for my boolean fields.
false
true
""
procedure.needsAuditing = Convert.ToBoolean(collection["needsAuditing"]);
How to I structure this variable so that if it is "" it will not try to convert it to a Boolean but instead pass null?

If you want a third state for a bool which indicates not determined(null) you can use a Nullable<bool>.
So change the property to:
public bool? needsAuditing{ get; set; }
and assign it in this way:
object needsAuditing = collection["needsAuditing"];
if(needsAuditing == null)
procedure.needsAuditing = (bool?) null;
else
procedure.needsAuditing = Convert.ToBoolean(needsAuditing);
Side-note: you should consider to use pascal case propertynames. See Property Naming Guidelines.

Try like this...first make procedure.needsAuditing Nullable see thisLink For More Details For Nullable Type.
then do it like this...
bool? c;
procedure.needsAuditing =collection["needsAuditing"]==""? c=null: Convert.ToBoolean(collection["needsAuditing"]);

if(string.IsNullOrEmpty(collection["needsAuditing"].ToString())
procedure.needsAuditing = null;
else
procedure.needsAuditing = Convert.ToBoolean(collection["needsAuditing"]);
Assuming needsAuditing is a bool?
Edit: I did an if else instead of a ? because the compiler would complain that there's no conversion between null and bool (which would be the return type of Convert.ToBoolean)

Create a ModelBinder which will convert your string input to nullable bool type.
Your method after that will look like this:
void Method(bool? value)
{
}

Assign the procedure.needsAuditing property to the result of the following method:
bool? ParseInput(string input)
{
int integerInput;
if (int.TryParse(input, out integerInput))
return integerInput == 1;
return null;
}

Is there a single-line way of casting an object to a decimal?

Is there a single-line way of casting an object to a decimal? data type?
My code looks something like this:
foreach(DataRow row in dt.Rows)
{
var a = new ClassA()
{
PropertyA = row["ValueA"] as decimal?,
PropertyB = row["ValueB"] as decimal?,
PropertyC = row["ValueC"] as decimal?
};
// Do something
}
However casting an object to a decimal? doesn't work the way I expect it to, and returns null every time.
The data rows are read from an Excel file, so the data type of the object in each row is either a double if there is a value, or a null string if it's left blank.
The suggested method to perform this cast is to use decimal.TryParse, however I do not want to create a temp variable for every decimal property on the class (I have about 7 properties on my actual class that are decimals, not 3).
decimal tmpvalue;
decimal? result = decimal.TryParse((string)value, out tmpvalue) ?
tmpvalue : (decimal?)null;
Is there a way I can cast a potentially null value to a decimal? in a single line?
I tried the answer posted here, however it doesn't appear to be working and gives me a null value as well because row["ValueA"] as string returns null.

Your best bet is to create an extension method. This is what I use, although you can tweak it to return a decimal? instead.
public static class ObjectExtensions
{
public static decimal ToDecimal(this object number)
{
decimal value;
if (number == null) return 0;
if (decimal.TryParse(number.ToString().Replace("$", "").Replace(",", ""), out value))
return value;
else
return 0;
}
public static decimal? ToNullableDecimal(this object number)
{
decimal value;
if (number == null) return null;
if (decimal.TryParse(number.ToString().Replace("$", "").Replace(",", ""), out value))
return value;
else
return null;
}
}
You would then use it by calling .ToDecimal() on any object, the same way you would call .ToString().
It's not one line, but it is only a single function call at the point where it's used, and it's very reusable.
Edited to add nullable version

providing there is a value it reads as a double, otherwise I think it's a string
Okay, so this complicates matters as sometimes you need to parse it and sometimes you don't.
The first thing we'll want to do is check if it's already a double and cast it, because casting is much cheaper than parsing; parsing is expensive.
Given that this logic will be non-trivial, it belongs in it's own method:
public static deciaml? getDecimal(object rawValue)
{
decimal finalValue;
double? doubleValue = rawValue as double?;
if(doubleValue.HasValue)
return (decimal) doubleValue.Value;
else if(decimal.TryParse(rawValue as string, out finalValue))
{
return finalValue;
}
else
{
return null;//could also throw an exception if you wanted.
}
}
If you know that it will always be a double, and you'll never want the string values, then it's easier still. In that case you need to cast it to a double, first, since that's it's real type, and then you can easily convert it to a decimal:
PropertyA = (decimal?)(row["ValueA"] as double?);

decimal.TryParse is the way to go. Just create a helper method and return the value. Pretty much the same code you mentioned:
private decimal? convertToNullableDecimal(object value){
decimal tmpvalue;
return decimal.TryParse((string)value, out tmpvalue) ? tmpvalue : (decimal?)null;
}
UPDATE
This assumes you need a nullable decimal back. Otherwise do as suggested in other answers, use the built-in method: Convert.ToDecimal

Try something like
Func<object, decimal?> l_convert =
(o) => decimal.TryParse((o ?? "").ToString(), out tmpvalue) ? tmpvalue : (decimal?)null;
var a = new ClassA()
{
PropertyA = l_convert(row["ValueA"]),
PropertyB = l_convert(row["ValueB"]),
PropertyC = l_convert(row["ValueC"])
};

Hope this helps
decimal temp;
foreach(DataRow row in dt.Rows)
{
var a = new ClassA()
{
PropertyA = Decimal.TryParse(row["ValueA"].ToString(), out temp) ? Convert.ToDecimal(row["ValueA"].ToString()) : (Decimal?) null,
....
};
// Do something
}

You may try the Decimal.Parse method.
public static decimal ToDecimal(this string value){
try
{
return Decimal.Parse(value);
}
catch (Exception)
{
// catch FormatException, NullException
return 0;
}
}
// use
PropertyA = row["ValueA"].ToString().ToDecimal();
PropertyB = row["ValueB"].ToString().ToDecimal();
PropertyC = row["ValueC"].ToString().ToDecimal();

Convert.ToDecimal(object) almost does what you want. It will convert the double value properly (without going to a string first), and also handle other types that can convert to decimal. The one thing it doesn't do well is the null conversion (or converting of String.Empty to null). This I would try to handle with an extension method.
public static class ObjectToDecimal
{
public static decimal? ToNullableDecimal(this object obj)
{
if(obj == null) return null;
if(obj is string && String.Empty.Equals(obj)) return null;
return Convert.ToDecimal(obj);
}
}
Just remember that Convert.ToDecimal(object) can thrown exceptions, so you'll have to watch out for that.

Return empty string if object null

The object i is from database.
PrDT is a string, PrDateTime is DataTimeOffset type, nullable
vi.PrDT = i.PrDateTime.Value.ToString("s");
What is the quick way?
I don't want if else etc...

Using the conditional operator:
vi.PrDT = i.PrDateTime.HasValue ? i.PrDateTime.Value.ToString("s") :
string.Empty;

You can do an extensions method:
public static class NullableToStringExtensions
{
public static string ToString<T>(this T? value, string format, string coalesce = null)
where T : struct, IFormattable
{
if (value == null)
{
return coalesce;
}
else
{
return value.Value.ToString(format, null);
}
}
}
and then:
vi.PrDT = i.PrDateTime.ToString("s", string.Empty);

string.Format("{0:s}", i.PrDateTime)
The above will return back an empty string if it's null. Since Nullable<T>.ToString checks for a null value and returns an empty string if it is, otherwise returns a string representation (but can't use formatting specifiers). Trick is to use string.Format, which allows you to use the format specifier you want (in this case, s) and still get the Nullable<T>.ToString behavior.

return (i.PrDateTime.Value ?? string.Empty).ToString();
Just tested and it seems to work.

return i.PrDateTime.Value.ToString("s") ?? string.Empty;