Essential Books

GroupBy Sum and Count

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Let’s take a sample class: 

public class Transaction
{
    public string Category { get; set; }
    public DateTime Date { get; set; }
    public decimal Amount { get; set; }
}

Now, let us consider a list of transactions: 

var transactions = new List<Transaction>
{
   new Transaction { Category = "Saving Account", Amount = 56, Date = DateTime.Today.AddDays(1) },
   new Transaction { Category = "Saving Account", Amount = 10, Date = DateTime.Today.AddDays(-10) },
   new Transaction { Category = "Credit Card", Amount = 15, Date = DateTime.Today.AddDays(1) },
   new Transaction { Category = "Credit Card", Amount = 56, Date = DateTime.Today },
   new Transaction { Category = "Current Account", Amount = 100, Date = DateTime.Today.AddDays(5) },
};

If you want to calculate category wise sum of amount and count, you can use GroupBy as follows: 

var summaryApproach1 = transactions.GroupBy(t => t.Category)
                           .Select(t => new
                           {
                               Category = t.Key,
                               Count = t.Count(),
                               Amount = t.Sum(ta => ta.Amount),
                           }).ToList();

Console.WriteLine("-- Summary: Approach 1 --");
summaryApproach1.ForEach(
            row => Console.WriteLine($"Category: {row.Category}, Amount: {row.Amount}, Count: {row.Count}"));

Alternatively, you can do this in one step: 

var summaryApproach2 = transactions.GroupBy(t => t.Category, (key, t) =>
{
        var transactionArray = t as Transaction[] ?? t.ToArray();
        return new
        {
            Category = key,
            Count = transactionArray.Length,
            Amount = transactionArray.Sum(ta => ta.Amount),
        };
}).ToList();

Console.WriteLine("-- Summary: Approach 2 --");
summaryApproach2.ForEach(
row => Console.WriteLine($"Category: {row.Category}, Amount: {row.Amount}, Count: {row.Count}"));

Output for both the above queries would be same:

Category: Saving Account, Amount: 66, Count: 2 Category: Credit Card, Amount: 71, Count: 2 Category: Current Account, Amount: 100, Count: 1

Live Demo in .NET Fiddle

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LINQ Queries:
* LINQ Queries
* Chaining methods
* First FirstOrDefault Last LastOrDefault Single and SingleOrDefault
* Except
* SelectMany
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* JOINS
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* Zip
* 09Defining a variable inside a Linq query let keyword
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* Query collection by type cast elements to type
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* Enumerating the Enumerable
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* Using Range with various Linq methods
* Sum
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* ToDictionary
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* Any and FirstOrDefault - best practice
* GroupBy one or multiple fields
* OrderBy
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* Joining multiple sequences
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* GroupBy Sum and Count
* Count and LongCount
* Linq Quantifiers
* Build your own Linq operators for IEnumerableT
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* Incrementally building a query
* Concat
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* Union
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* Select with FuncTSource int TResult selector - Use to get ranking of elements
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0 Getting Started With C#
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4 Equality Operator
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118 Task Parallel Library TPL Dataflow Constructs
119 Functional Programming
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125 Accessing Databases
126 Using SQLite
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132 Implementing Decorating Design Pattern
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146 Unsafe Code in .NET
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151 Creating Own MessageBox in Windows Form Application
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153 Garbage Collector
154 Windows Communication Foundation
155 Contributors