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Elegant way to query a dictionary in C#

I am trying to create an elegant and extensible way of querying a dictionary which maps an enum to a set of strings.
So I have this class SearchFragments that has the dictionary in it. I then want a method wherein consumers of this class can simply ask "HasAny" and, this is the bit where I am struggling, simply pass in some query like expression and get the boolean answer back.
public class SearchFragments
{
private readonly IDictionary<SearchFragmentEnum, IEnumerable<string>> _fragments;
public SearchFragments()
{
_fragments = new Dictionary<SearchFragmentEnum, IEnumerable<string>>();
}
public bool HasAny(IEnumerable<SearchFragmentEnum> of)
{
int has = 0;
_fragments.ForEach(x => of.ForEach(y => has += x.Key == y ? 1 : 0));
return has >= 1;
}
}
The problem with the way this currently is, is that consumers of this class now have to construct an IEnumerable<SearchFragmentEnum> which can be quite messy.
What I am looking for is that the consuming code will be able to write something along the lines of:
searchFragments.HasAny(SearchFragmentEnum.Name, SearchFragmentEnum.PhoneNumber)
But where that argument list can vary in size (without me having to write method overloads in the SearchFragments class for every possible combination (such that if new values are added to the SearchFragmentEnum at a future date I won't have to update the class.

You can use params[]
public bool HasAny(params SearchFragmentEnum[] of)
{ ...
Sidenote: you know that LIN(Q) queries should just query a source and never cause any side-effects? But your query does unnecessarily increment the integer:
_fragments.ForEach(x => of.ForEach(y => has += x.Key == y ? 1 : 0));
Instead use this (which is also more efficient and more readable):
return _fragments.Keys.Intersect(of).Any();
An even more efficient alternative to this is Sergey's idea:
return of?.Any(_fragments.ContainsKey) == true;

For variable sized arguments in c# you use the params keyword:
public int HasAny(params SearchFragmentEnum[] of)
The .Net API usually offers a couple of overloads of this for performance reasons; the parameters passed are copied into a new array. Explicitely providing overloads for the most common cases avoids this.
public int HasAny(SearchfragmentEnum of1)
public int HasAny(SearchFragmentEnum of1, SearchFragmentEnum of2)
etc.
Instead of using params you could also consider marking your enum with the [Flags] attribute. Parameters could than be passed like HasAny(SearchFragmentEnum.Name | SearchFragmentEnum.PhoneNumber. Examples abundant on StackOverflow (e.g. Using a bitmask in C#)

Use the params keyword to allow a varying number of arguments. Further, you can simplify your code by looping over the smaller of array. Also, you are using a dictionary that has O(1) key check, so it is uneccessary to have an inner loop:
public bool HasAny(params SearchFragmentEnum[] of)
{
foreach(var o in of) {
if (this._fragments.ContainsKey(o))
return true;
}
return false;
}
or shorter with LINQ
public bool HasAny(params SearchFragmentEnum[] of) {
return of?.Any(_fragments.ContainsKey) ?? false;
}

Best way to remove the last character from a string built with stringbuilder

I have the following
data.AppendFormat("{0},",dataToAppend);
The problem with this is that I am using it in a loop and there will be a trailing comma. What is the best way to remove the trailing comma?
Do I have to change data to a string and then substring it?

The simplest and most efficient way is to perform this command:
data.Length--;
by doing this you move the pointer (i.e. last index) back one character but you don't change the mutability of the object. In fact, clearing a StringBuilder is best done with Length as well (but do actually use the Clear() method for clarity instead because that's what its implementation looks like):
data.Length = 0;
again, because it doesn't change the allocation table. Think of it like saying, I don't want to recognize these bytes anymore. Now, even when calling ToString(), it won't recognize anything past its Length, well, it can't. It's a mutable object that allocates more space than what you provide it, it's simply built this way.

Just use
string.Join(",", yourCollection)
This way you don't need the StringBuilder and the loop.
Long addition about async case. As of 2019, it's not a rare setup when the data are coming asynchronously.
In case your data are in async collection, there is no string.Join overload taking IAsyncEnumerable<T>. But it's easy to create one manually, hacking the code from string.Join:
public static class StringEx
{
public static async Task<string> JoinAsync<T>(string separator, IAsyncEnumerable<T> seq)
{
if (seq == null)
throw new ArgumentNullException(nameof(seq));
await using (var en = seq.GetAsyncEnumerator())
{
if (!await en.MoveNextAsync())
return string.Empty;
string firstString = en.Current?.ToString();
if (!await en.MoveNextAsync())
return firstString ?? string.Empty;
// Null separator and values are handled by the StringBuilder
var sb = new StringBuilder(256);
sb.Append(firstString);
do
{
var currentValue = en.Current;
sb.Append(separator);
if (currentValue != null)
sb.Append(currentValue);
}
while (await en.MoveNextAsync());
return sb.ToString();
}
}
}
If the data are coming asynchronously but the interface IAsyncEnumerable<T> is not supported (like the mentioned in comments SqlDataReader), it's relatively easy to wrap the data into an IAsyncEnumerable<T>:
async IAsyncEnumerable<(object first, object second, object product)> ExtractData(
SqlDataReader reader)
{
while (await reader.ReadAsync())
yield return (reader[0], reader[1], reader[2]);
}
and use it:
Task<string> Stringify(SqlDataReader reader) =>
StringEx.JoinAsync(
", ",
ExtractData(reader).Select(x => $"{x.first} * {x.second} = {x.product}"));
In order to use Select, you'll need to use nuget package System.Interactive.Async. Here you can find a compilable example.

How about this..
string str = "The quick brown fox jumps over the lazy dog,";
StringBuilder sb = new StringBuilder(str);
sb.Remove(str.Length - 1, 1);

Use the following after the loop.
.TrimEnd(',')
or simply change to
string commaSeparatedList = input.Aggregate((a, x) => a + ", " + x)

I prefer manipulating the length of the stringbuilder:
data.Length = data.Length - 1;

I recommend, you change your loop algorithm:
Add the comma not AFTER the item, but BEFORE
Use a boolean variable, that starts with false, do suppress the first comma
Set this boolean variable to true after testing it

You should use the string.Join method to turn a collection of items into a comma delimited string. It will ensure that there is no leading or trailing comma, as well as ensure the string is constructed efficiently (without unnecessary intermediate strings).

The most simple way would be to use the Join() method:
public static void Trail()
{
var list = new List<string> { "lala", "lulu", "lele" };
var data = string.Join(",", list);
}
If you really need the StringBuilder, trim the end comma after the loop:
data.ToString().TrimEnd(',');

Yes, convert it to a string once the loop is done:
String str = data.ToString().TrimEnd(',');

You have two options. First one is very easy use Remove method it is quite effective. Second way is to use ToString with start index and end index (MSDN documentation)

Similar SO question here.
I liked the using a StringBuilder extension method.
RemoveLast Method

Gotcha!!
Most of the answers on this thread won't work if you use AppendLine like below:
var builder = new StringBuilder();
builder.AppendLine("One,");
builder.Length--; // Won't work
Console.Write(builder.ToString());
builder = new StringBuilder();
builder.AppendLine("One,");
builder.Length += -1; // Won't work
Console.Write(builder.ToString());
builder = new StringBuilder();
builder.AppendLine("One,");
Console.Write(builder.TrimEnd(',')); // Won't work
Fiddle Me
WHY??? #(&**(&#!!
The issue is simple but took me a while to figure it out: Because there are 2 more invisible characters at the end CR and LF (Carriage Return and Line Feed). Therefore, you need to take away 3 last characters:
var builder = new StringBuilder();
builder.AppendLine("One,");
builder.Length -= 3; // This will work
Console.WriteLine(builder.ToString());
In Conclusion
Use Length-- or Length -= 1 if the last method you called was Append. Use Length =- 3 if you the last method you called AppendLine.

Simply shortens the stringbuilder length by 1;
StringBuilder sb = new StringBuilder();
sb.Length--;
i know this is not the effective way as it translates to sb = sb-1;
Alternative Effective solution
sb.Remove(starting_index, how_many_character_to_delete);
for our case it would be
sb.Remove(sb.length-1,1)

best way to convert collection to string

I need to convert a collection of <string,string> to a single string containing all the values in the collection like KeyValueKeyValue... But How do I do this effectively?
I have done it this way at the moment:
parameters = string.Join("", requestParameters.Select(x => string.Concat(x.Key, x.Value)));
But not sure it is the best way to do it, would a string builder be better? I guess the collection will contain a max of 10 pairs.

string.Join used to not really be the best option since it only accepted string[] or object[] parameters, requiring that any select-style queries needed to be completely evaluated and put into an array first.
.NET 4.0 brought with it an overload that accepts IEnumerable<string> -- which is what you are using -- and even an overload that accepts any IEnumerable<T>. These are definitely your best bet as they are now part of the BCL.
Incidentally, cracking open the source for the first overload in Reflector shows code that follows pretty closely to what davisoa suggested:
public static string Join(string separator, IEnumerable<string> values)
{
if (values == null)
{
throw new ArgumentNullException("values");
}
if (separator == null)
{
separator = Empty;
}
using (IEnumerator<string> enumerator = values.GetEnumerator())
{
if (!enumerator.MoveNext())
{
return Empty;
}
StringBuilder builder = new StringBuilder();
if (enumerator.Current != null)
{
builder.Append(enumerator.Current);
}
while (enumerator.MoveNext())
{
builder.Append(separator);
if (enumerator.Current != null)
{
builder.Append(enumerator.Current);
}
}
return builder.ToString();
}
}
So in other words, if you were to change this code to use a StringBuilder, you'd just be rewriting what MS already wrote for you.

With such a small collection, there isn't much of a performance concern, but I would probably just use a StringBuilder to Append all of the values.
Like this:
var sb = new Text.StringBuilder;
foreach (var item in requestParameters)
{
sb.AppendFormat("{0}{1}", item.Key, item.Value);
}
var parameters = sb.ToString();

String builder would be fine. Use append to add each a string to the string builder.
Basically the only reason why concat, replace, join, string+string , etc are considered not-the-best because they all tend to destroy the current string & recreate a new one.
So when you have adding strings like upto 10-12 time it really means you will destroy & recreate a string that many times.

Can all 'for' loops be replaced with a LINQ statement?

Is it possible to write the following 'foreach' as a LINQ statement, and I guess the more general question can any for loop be replaced by a LINQ statement.
I'm not interested in any potential performance cost just the potential of using declarative approaches in what is traditionally imperative code.
private static string SomeMethod()
{
if (ListOfResources .Count == 0)
return string.Empty;
var sb = new StringBuilder();
foreach (var resource in ListOfResources )
{
if (sb.Length != 0)
sb.Append(", ");
sb.Append(resource.Id);
}
return sb.ToString();
}
Cheers
AWC

Sure. Heck, you can replace arithmetic with LINQ queries:
http://blogs.msdn.com/ericlippert/archive/2009/12/07/query-transformations-are-syntactic.aspx
But you shouldn't.
The purpose of a query expression is to represent a query operation. The purpose of a "for" loop is to iterate over a particular statement so as to have its side-effects executed multiple times. Those are frequently very different. I encourage replacing loops whose purpose is merely to query data with higher-level constructs that more clearly query the data. I strongly discourage replacing side-effect-generating code with query comprehensions, though doing so is possible.

In general yes, but there are specific cases that are extremely difficult. For instance, the following code in the general case does not port to a LINQ expression without a good deal of hacking.
var list = new List<Func<int>>();
foreach ( var cur in (new int[] {1,2,3})) {
list.Add(() => cur);
}
The reason why is that with a for loop, it's possible to see the side effects of how the iteration variable is captured in a closure. LINQ expressions hide the lifetime semantics of the iteration variable and prevent you from seeing side effects of capturing it's value.
Note. The above code is not equivalent to the following LINQ expression.
var list = Enumerable.Range(1,3).Select(x => () => x).ToList();
The foreach sample produces a list of Func<int> objects which all return 3. The LINQ version produces a list of Func<int> which return 1,2 and 3 respectively. This is what makes this style of capture difficult to port.

In fact, your code does something which is fundamentally very functional, namely it reduces a list of strings to a single string by concatenating the list items. The only imperative thing about the code is the use of a StringBuilder.
The functional code makes this much easier, actually, because it doesn’t require a special case like your code does. Better still, .NET already has this particular operation implemented, and probably more efficient than your code1):
return String.Join(", ", ListOfResources.Select(s => s.Id.ToString()).ToArray());
(Yes, the call to ToArray() is annoying but Join is a very old method and predates LINQ.)
Of course, a “better” version of Join could be used like this:
return ListOfResources.Select(s => s.Id).Join(", ");
The implementation is rather straightforward – but once again, using the StringBuilder (for performance) makes it imperative.
public static String Join<T>(this IEnumerable<T> items, String delimiter) {
if (items == null)
throw new ArgumentNullException("items");
if (delimiter == null)
throw new ArgumentNullException("delimiter");
var strings = items.Select(item => item.ToString()).ToList();
if (strings.Count == 0)
return string.Empty;
int length = strings.Sum(str => str.Length) +
delimiter.Length * (strings.Count - 1);
var result = new StringBuilder(length);
bool first = true;
foreach (string str in strings) {
if (first)
first = false;
else
result.Append(delimiter);
result.Append(str);
}
return result.ToString();
}
1) Without having looked at the implementation in the reflector, I’d guess that String.Join makes a first pass over the strings to determine the overall length. This can be used to initialize the StringBuilder accordingly, thus saving expensive copy operations later on.
EDIT by SLaks: Here is the reference source for the relevant part of String.Join from .Net 3.5:
string jointString = FastAllocateString( jointLength );
fixed (char * pointerToJointString = &jointString.m_firstChar) {
UnSafeCharBuffer charBuffer = new UnSafeCharBuffer( pointerToJointString, jointLength);
// Append the first string first and then append each following string prefixed by the separator.
charBuffer.AppendString( value[startIndex] );
for (int stringToJoinIndex = startIndex + 1; stringToJoinIndex <= endIndex; stringToJoinIndex++) {
charBuffer.AppendString( separator );
charBuffer.AppendString( value[stringToJoinIndex] );
}
BCLDebug.Assert(*(pointerToJointString + charBuffer.Length) == '\0', "String must be null-terminated!");
}

The specific loop in your question can be done declaratively like this:
var result = ListOfResources
.Select<Resource, string>(r => r.Id.ToString())
.Aggregate<string, StringBuilder>(new StringBuilder(), (sb, s) => sb.Append(sb.Length > 0 ? ", " : String.Empty).Append(s))
.ToString();
As to performance, you can expect a performance drop but this is acceptable for most applications.

I think what's most important here is that to avoid semantic confusion, your code should only be superficially functional when it is actually functional. In other words, please don't use side effects in LINQ expressions.

Technically, yes.
Any foreach loop can be converted to LINQ by using a ForEach extension method,such as the one in MoreLinq.
If you only want to use "pure" LINQ (only the built-in extension methods), you can abuse the Aggregate extension method, like this:
foreach(type item in collection { statements }
type item;
collection.Aggregate(true, (j, itemTemp) => {
item = itemTemp;
statements
return true;
);
This will correctly handle any foreach loop, even JaredPar's answer. EDIT: Unless it uses ref / out parameters, unsafe code, or yield return.
Don't you dare use this trick in real code.
In your specific case, you should use a string Join extension method, such as this one:
///<summary>Appends a list of strings to a StringBuilder, separated by a separator string.</summary>
///<param name="builder">The StringBuilder to append to.</param>
///<param name="strings">The strings to append.</param>
///<param name="separator">A string to append between the strings.</param>
public static StringBuilder AppendJoin(this StringBuilder builder, IEnumerable<string> strings, string separator) {
if (builder == null) throw new ArgumentNullException("builder");
if (strings == null) throw new ArgumentNullException("strings");
if (separator == null) throw new ArgumentNullException("separator");
bool first = true;
foreach (var str in strings) {
if (first)
first = false;
else
builder.Append(separator);
builder.Append(str);
}
return builder;
}
///<summary>Combines a collection of strings into a single string.</summary>
public static string Join<T>(this IEnumerable<T> strings, string separator, Func<T, string> selector) { return strings.Select(selector).Join(separator); }
///<summary>Combines a collection of strings into a single string.</summary>
public static string Join(this IEnumerable<string> strings, string separator) { return new StringBuilder().AppendJoin(strings, separator).ToString(); }

In general, you can write a lambda expression using a delegate which represents the body of a foreach cycle, in your case something like :
resource => { if (sb.Length != 0) sb.Append(", "); sb.Append(resource.Id); }
and then simply use within a ForEach extension method. Whether this is a good idea depends on the complexity of the body, in case it's too big and complex you probably don't gain anything from it except for possible confusion ;)

Good way to concatenate string representations of objects?

Ok,
We have a lot of where clauses in our code. We have just as many ways to generate a string to represent the in condition. I am trying to come up with a clean way as follows:
public static string Join<T>(this IEnumerable<T> items, string separator)
{
var strings = from item in items select item.ToString();
return string.Join(separator, strings.ToArray());
}
it can be used as follows:
var values = new []{1, 2, 3, 4, 5, 6};
values.StringJoin(",");
// result should be:
// "1,2,3,4,5,6"
So this is a nice extension method that does a very basic job. I know that simple code does not always turn into fast or efficient execution, but I am just curious as to what could I have missed with this simple code. Other members of our team are arguing that:
it is not flexible enough (no control of the string representation)
may not be memory efficient
may not be fast
Any expert to chime in?
Regards,
Eric.

Regarding the first issue, you could add another 'formatter' parameter to control the conversion of each item into a string:
public static string Join<T>(this IEnumerable<T> items, string separator)
{
return items.Join(separator, i => i.ToString());
}
public static string Join<T>(this IEnumerable<T> items, string separator, Func<T, string> formatter)
{
return String.Join(separator, items.Select(i => formatter(i)).ToArray());
}
Regarding the second two issues, I wouldn't worry about it unless you later run into performance issues and find it to be a problem. It's unlikely to much of a bottleneck however...

For some reason, I thought that String.Join is implemented in terms of a StringBuilder class. But if it isn't, then the following is likely to perform better for large inputs since it doesn't recreate a String object for each join in the iteration.
public static string Join<T>(this IEnumerable<T> items, string separator)
{
// TODO: check for null arguments.
StringBuilder builder = new StringBuilder();
foreach(T t in items)
{
builder.Append(t.ToString()).Append(separator);
}
builder.Length -= separator.Length;
return builder.ToString();
}
EDIT: Here is an analysis of when it is appropriate to use StringBuilder and String.Join.

Why don't you use StringBuilder, and iterate through the collection yourself, appending.
Otherwise you are creating an array of strings (var strings) and then doing the Join.

You are missing null checks for the sequence and the items of the sequence. And yes, it is not the fastest and most memory efficient way. One would probably just enumerate the sequence and render the string representations of the items into a StringBuilder. But does this really matter? Are you experiencing performance problems? Do you need to optimize?

this would work also:
public static string Test(IEnumerable<T> items, string separator)
{
var builder = new StringBuilder();
bool appendSeperator = false;
if(null != items)
{
foreach(var item in items)
{
if(appendSeperator)
{
builder.Append(separator)
}
builder.Append(item.ToString());
appendSeperator = true;
}
}
return builder.ToString();
}