IEnumerable < >洗牌

我用Fisher-Yates算法实现了一个shuffle函数。

通过使用yield return和将代码分解为两个函数，它实现了适当的参数验证和延迟执行。(谢谢丹，在我的第一个版本中指出了这个缺陷)

static public IEnumerable<T> Shuffle<T>(this IEnumerable<T> source)
{
    if (source == null) throw new ArgumentNullException("source");

    return ShuffleIterator(source);
}

static private IEnumerable<T> ShuffleIterator<T>(this IEnumerable<T> source)
{
    T[] array = source.ToArray();
    Random rnd = new Random();          
    for (int n = array.Length; n > 1;)
    {
        int k = rnd.Next(n--); // 0 <= k < n

        //Swap items
        if (n != k)
        {
            T tmp = array[k];
            array[k] = array[n];
            array[n] = tmp;
        }
    }

    foreach (var item in array) yield return item;
}

For adding multiple elements to a collection that doesn't have AddRange, e.g., collection.Add(item1, item2, itemN); static void Add<T>(this ICollection<T> coll, params T[] items) { foreach (var item in items) coll.Add(item); } The following is like string.Format() but with custom string representation of arguments, e.g., "{0} {1} {2}".Format<Custom>(c=>c.Name,"string",new object(),new Custom()) results in "string {System.Object} Custom1Name" static string Format<T>( this string format , Func<T,object> select , params object[] args) { for(int i=0; i < args.Length; ++i) { var x = args[i] as T; if (x != null) args[i] = select(x); } return string.Format(format, args); }

// Values ordered true/false
// True/false values separated by a capital letter
// Only two values allowed
// ---------------------------
// Limited, but could be useful
public enum BooleanFormat
{
    OneZero,
    YN,
    YesNo,
    TF,
    TrueFalse,
    PassFail,
    YepNope
}

public static class BooleanExtension
{
    /// <summary>
    /// Converts the boolean value of this instance to the specified string value. 
    /// </summary>
    private static string ToString(this bool value, string passValue, string failValue)
    {
        return value ? passValue : failValue;
    }

    /// <summary>
    /// Converts the boolean value of this instance to a string. 
    /// </summary>
    /// <param name="booleanFormat">A BooleanFormat value. 
    /// Example: BooleanFormat.PassFail would return "Pass" if true and "Fail" if false.</param>
    /// <returns>Boolean formatted string</returns>
    public static string ToString(this bool value, BooleanFormat booleanFormat)
    {
        string booleanFormatString = Enum.GetName(booleanFormat.GetType(), booleanFormat);
        return ParseBooleanString(value, booleanFormatString);      
    }

    // Parses boolean format strings, not optimized
    private static string ParseBooleanString(bool value, string booleanFormatString)
    {
        StringBuilder trueString = new StringBuilder();
        StringBuilder falseString = new StringBuilder();

        int charCount = booleanFormatString.Length;

        bool isTrueString = true;

        for (int i = 0; i != charCount; i++)
        {
            if (char.IsUpper(booleanFormatString[i]) && i != 0)
                isTrueString = false;

            if (isTrueString)
                trueString.Append(booleanFormatString[i]);
            else
                falseString.Append(booleanFormatString[i]);
        }

        return (value == true ? trueString.ToString() : falseString.ToString());
    }

我创建了一个扩展方法来在下拉菜单中选择一个项目。

下面是代码

 public static class Utilities
{
    public enum DropDownListSelectionType
    {
        ByValue,
        ByText
    }

    public static void SelectItem(this  System.Web.UI.WebControls.DropDownList drp, string selectedValue, DropDownListSelectionType type)
    {
        drp.ClearSelection();
        System.Web.UI.WebControls.ListItem li;
        if (type == DropDownListSelectionType.ByValue)
            li = drp.Items.FindByValue(selectedValue.Trim());
        else
            li = drp.Items.FindByText(selectedValue.Trim());
        if (li != null)
            li.Selected = true;
    }}

此方法可以由以下代码行调用，以按文本进行选择

DropDownList1.SelectItem("ABCD", Utilities.DropDownListSelectionType.ByText);

或者按值选择

DropDownList1.SelectItem("11", Utilities.DropDownListSelectionType.ByValue);

上面的代码不选择任何东西，如果它不能找到传递进来的文本/值。

处理大小的简便方法:

public static class Extensions {
    public static int K(this int value) {
        return value * 1024;
    }
    public static int M(this int value) {
        return value * 1024 * 1024;
    }
}

public class Program {
    public void Main() {
        WSHttpContextBinding serviceMultipleTokenBinding = new WSHttpContextBinding() {
            MaxBufferPoolSize = 2.M(), // instead of 2097152
            MaxReceivedMessageSize = 64.K(), // instead of 65536
        };
    }
}

Sql server有~2000个参数的限制，如果你有10k个id并想要与它们连接的记录，这是一个痛苦。我写了这些方法，接受批量id列表，并像这样调用:

List<Order> orders = dataContext.Orders.FetchByIds(
  orderIdChunks,
  list => row => list.Contains(row.OrderId)
);

List<Customer> customers = dataContext.Orders.FetchByIds(
  orderIdChunks,
  list => row => list.Contains(row.OrderId),
  row => row.Customer
);

public static List<ResultType> FetchByIds<RecordType, ResultType>(
    this IQueryable<RecordType> querySource,
    List<List<int>> IdChunks,
    Func<List<int>, Expression<Func<RecordType, bool>>> filterExpressionGenerator,
    Expression<Func<RecordType, ResultType>> projectionExpression
    ) where RecordType : class
{
    List<ResultType> result = new List<ResultType>();
    foreach (List<int> chunk in IdChunks)
    {
        Expression<Func<RecordType, bool>> filterExpression =
            filterExpressionGenerator(chunk);

        IQueryable<ResultType> query = querySource
            .Where(filterExpression)
            .Select(projectionExpression);

        List<ResultType> rows = query.ToList();
        result.AddRange(rows);
    }

    return result;
}

public static List<RecordType> FetchByIds<RecordType>(
    this IQueryable<RecordType> querySource,
    List<List<int>> IdChunks,
    Func<List<int>, Expression<Func<RecordType, bool>>> filterExpressionGenerator
    ) where RecordType : class
{
    Expression<Func<RecordType, RecordType>> identity = r => r;

    return FetchByIds(
        querySource,
        IdChunks,
        filterExpressionGenerator,
        identity
        );
}