GroupBy Sum and Count C# with Example

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 
{ 
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