ienumerable

问题 In general, I am using a List and then returning them as IEnumerable when I no longer need to update them. However, I ran into an issue where I actually need to enumerate through them but first need to know the count. Will IEnumerable enumerate every item and find the count (O(N)), or will it rely on List 's Count property (O(1))? Also, what if the IEnumerable is the result of a LINQ query? 回答1: Will IEnumerable enumerate every item and find the count (O(N)), or will it rely on List's Count

问题 I had a List<> of my objects, but now need to change it to an IEnumerable<>. So, I have this: public IEnumerable<TransactionSplitLine> TransactionSplitLines { get; set; } However, I can no longer do: reply.TransactionSplitLines.Add(new TransactionSplitLine {Amount = "100", Category = "Test", SubCategory = "Test More", CategoryId=int.Parse(c)}); How should I be adding items now? 回答1: You could do something like the following, using Concat: reply.TransactionSplitLines = reply

问题 I have this two errors: 'System.Array' does not contain a definition for 'Aggregate' and no extension method 'Aggregate' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?) 'System.Collections.Generic.IEnumerable<object[]>' does not contain a definition for 'ToArray' and no extension method 'ToArray' accepting a first argument of type 'System.Collections.Generic.IEnumerable<object[]>' could be found (are you missing a

问题 Consider the following code: List<int> list = new List<int>(); IEnumerable<int> enumerable = list; IEnumerator<int> enumerator = enumerable.GetEnumerator(); list.Add(1); bool any = enumerator.MoveNext(); At runtime, the last line throws an: InvalidOperationException: Collection was modified; enumeration operation may not execute. I understand the need for IEnumerators to throw 'Collection was modified' exceptions when the IEnumerable changes, but I don't understand this: Why does the

问题 Consider the following code: List<int> list = new List<int>(); IEnumerable<int> enumerable = list; IEnumerator<int> enumerator = enumerable.GetEnumerator(); list.Add(1); bool any = enumerator.MoveNext(); At runtime, the last line throws an: InvalidOperationException: Collection was modified; enumeration operation may not execute. I understand the need for IEnumerators to throw 'Collection was modified' exceptions when the IEnumerable changes, but I don't understand this: Why does the

问题 I am using a dictionary wrapper class, and I want to iterate through it using key-value pairs as seen below private void LoadVariables(LogDictionary dic) { foreach (var entry in dic) { _context.Variables[entry.Key] = entry.Value; } } but a NotImplementedException is thrown because I did not implement the GetEnumerator() method. Here is my wrapper class: public class LogDictionary: IDictionary<String, object> { DynamicTableEntity _dte; public LogDictionary(DynamicTableEntity dte) { _dte = dte;

问题 I am using a dictionary wrapper class, and I want to iterate through it using key-value pairs as seen below private void LoadVariables(LogDictionary dic) { foreach (var entry in dic) { _context.Variables[entry.Key] = entry.Value; } } but a NotImplementedException is thrown because I did not implement the GetEnumerator() method. Here is my wrapper class: public class LogDictionary: IDictionary<String, object> { DynamicTableEntity _dte; public LogDictionary(DynamicTableEntity dte) { _dte = dte;

问题 I have an IEnumerable of Lesson objects: IEnumerable<Lesson> filteredLessons I convert it to a List through the following method: ToList(); But I want the returned list to contain only the first property, lessonid , not all the Lesson properties. How can I get the data of specific property of the list instead of the objects? 回答1: You can select the value you want first, like this: filteredLessons.Select(l => l.lessonId).ToList(); And you'll get a list of ID's 回答2: If you want to get the the

问题 There is a Contains() extension method on IEnumerable; In VB I am able to do this: If New String() {"A", "B"}.Contains("B") Then ' ... End If What would be the equivalent of this in C#? 回答1: It is the same idea in C#, using LINQ extension methods: using System.Linq; ... if (new string[] {"A", "B"}.Contains("B")) { ... } 来源： https://stackoverflow.com/questions/1972820/use-contains-extension-method-on-arrays-in-c-sharp

问题 I am trying to check whether IEnumerable<> is null or empty but somehow my if check always fails whenever it is empty. private bool Update() { IEnumerable<RecordHolder> recordHolders = GetData(); // below check doesn't work if (recordHolders == null || !recordHolders.Any()) return false; // .. some other code } public IEnumerable<RecordHolder> GetData() { var isSuccess = PullRemote(url); if (!isSuccess.Result) { yield return default; } // .. some other code } Whenever my GetData() method