如何简化 C# 中的大 if 语句？

Question

Current in my code, if statement is too big and many repetitions.

if (!String.Equals(node.Name, "A", StringComparison.OrdinalIgnoreCase)
 && !String.Equals(node.Name, "B", StringComparison.OrdinalIgnoreCase)
 && !String.Equals(node.Name, "C", StringComparison.OrdinalIgnoreCase)
 &&  ... 

I would like to make it concise. But my idea is only to create a function and passing List or Array as param.

List<String> target = new List<String>() { "A", "B", "C" ... };
MultipleStringCompare(node.Name, target, , StringComparison.OrdinalIgnoreCase);

Is there any more beautiful way to make it simple without making any function in modern C# syntax?

Answer 1

The fastest will be a HashSet<string> . This becomes more evident as you get into the hundreds or thousands or more of strings to compare against. Think a list of profane words, or a list of customer email addresses, etc. Unlike a one-off LINQ query with a temporary list or array, the set you create will be queried super fast and can be mutated if you desire to add even more words at runtime if you want. You could wrap this in a function, but doing a strings.Contains(...) is so easy it may not be worth it.

The following snippet also takes care of making every comparison case-insensitive so you don't have to think about it every time you do a string compare.

// create a fast lookup for strings using a case-insensitive set
HashSet<string> strings = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { "A", "B", "C", "D" };
if (strings.Contains(node.Name))
{
    // string exists, do logic...
}

// later you can add or remove strings if you want...
// strings.Add("E");
// strings.Remove("A");

Answer 2

Yep, there's a way to do that using LINQ's functions.

The All function checks if all elements match the condition.

The Any function checks if at least one element matches the condition.

So in your case, your code will look like this

var result = !target.Any(str => string.Equals(node.Name, str, StringComparison.OrdinalIgnoreCase));

Answer 3

You can use a LINQ statement:

string[] comparisons = { "A", "B", "C" ... };
bool noMatch = comparisons.All(n => !String.Equals(node.Name, n, StringComparison.OrdinalIgnoreCase));

If noMatch is true, this is equivalent to your if statement being true. This assumes the strings you're comparing to are in the comparisons array.

How this works:

.All() is a LINQ extension method that takes a predicate as an input. This function will return true if the passed predicate returns true for every iteration. The predicate takes a single string and returns a boolean. So this is effectively equivalent to:

bool noMatch = true;
foreach (string n in comparisons)
{
    if (!String.Equals(node.Name, n, StringComparison.OrdinalIgnoreCase))
        continue;
    noMatch = false;
    break;
}

Answer 4

You can use the Any method in LINQ:

string s = "A";
!(new List<string> { "A", "B", "C", "D" }).Any(x => String.Equals(x, s, StringComparison.OrdinalIgnoreCase)

Answer 5

You can do:

var toAvoid = new []{"A", "B", "C"};
var isNameNotInToAvoid = !toAvoid.Contains(node.Name,
    StringComparer.InvariantCultureIgnoreCase);

if(isNameNotInToAvoid)

Answer 6

Probably simplest:

if ("ABCDEFG".Contains(node.Name.ToUpper()))
    ;// things to do if node.Name is in string "ABCDEFG"

or, in case of not only single characters comparison:

if ("AB|CDE|F|G|HIJK|L".Split('|').Any(c => String.Equals(node.Name, c.ToString(), StringComparison.OrdinalIgnoreCase)))
    ;// things to do if node.Name is one of "AB", "CDE", "F", "G", "HIJK", "L"