是否有内置的 C# 方法将 go 从空字符串转换为 null 值？

Question

There are many times in which I have an input text, and, if it's empty (if the user didn't type any text, for example), I want to send a null value to the DB query.

I do not want to send String.Empty. (or "" ).

Thus I find myself doing this a lot:

var mySqlValue =  string.IsNullOrEmpty( tbCustomerId.Text)?null:tbCustomerId.Text;

This seems ugly to me. .NET gives a lot of other solutions for the opposite scenarios:

string.IsNullOrEmpty
string.IsNullOrWhiteSpace
myProblemVal ?? myDefultVal

Is there anything built-in to C# that lets me do this a shorter way, like the opposite direction?

I know this can be solved by writing my own extension methods and I know how to do that; that's not a solution I'm interested in.

I'm looking for some code like " if empty -> null ".

Answer 1

You can use an extension method:

public static class Extensions {
    public static string NullIfWhiteSpace(this string value) {
        if (String.IsNullOrWhiteSpace(value)) { return null; }
        return value;
    }
}

Which you could use like that:

var mySqlValue = tbCustomerId.Text.NullIfWhiteSpace();

I don't really know what you imagine by something better than Extension methods. How do you define "better"? Shorter? Using a special keyword? Using rarely used operators to look clever? This is already just a single method call appended to your value, which even works on null values, and the logic you need can't really be expressed in a shorter way than this. Also, I don't know of any special syntax for it.

Answer 2

but is there anything better ?

No, although I'd argue that what you describe in the question (extension methods) is absolutely fine. And as you describe in the question, in the other direction you have null-coalescing.

Answer 3

Declare your own static method :

public static string NullIfEmpty(string value)
{
  return string.IsNullOrEmpty(value) ? null : value;
}

Doing

MyHelper.NullIfEmpty(value) is not uglier than a call to a static method of string type... And it seems cleaner than writing "string.IsNullOrEmpty( tbCustomerId.Text)?null:tbCustomerId.Text" each time.

Answer 4

Starting with your string

string myString = "";

Write a method that converts blank to null

public static string NullIf(string value)
{
    if (String.IsNullOrWhiteSpace(value)) { return null; }
    return value;
}

Then wrap in the method and call it back with the same syntax you'd expect in SQL

NullIf(myString);

Answer 5

Is it not what you directly asked, but one way to achieve what you are trying to do (pass a null value to SQL for a string if the string's value is empty) is use an if-statement to check the length of the string value being passed during the SQL Query parameter assignment (eg Parameters.AddWithValue("@parameter", mySqlValue) ):

if (mySqlValue.Length > 0) {
    SqlCommand.Parameters.AddWithValue("@mySqlValue", mySqlValue);
}
else {
    SqlCommand.Parameters.AddWithValue("@mySqlValue", DBNull.Value);
}

where SqlCommand is the variable in a using statement that utilizes a parameterized SQL Query and a pre-established SQL connection/connection string.

It's not an elegant solution, but it works (and doesn't require that you write an extension method).

Answer 6

If you want to return an empty string as null automatically when deserializing json:

public partial class MyClass
{
    string sample;
    [JsonProperty("myProperty")]
    public string MyProperty
    {
        get { return sample; }
        set { sample = string.IsNullOrEmpty(value) ? null : value; }
    }
}