Using Linq .Min() - At least one object must implement IComparable

Question

I want to get the smallest value in a collection and I am using LINQ to do this. I've read online I can use Min() but I am getting the error message:

 At least one object must implement IComparable

This is my code

public virtual void GetBestValue(ModelEnergyCalculator ModelEnergyCalculator)
{
     ModelTariffQuote ModelTariffQuote = (from q in ModelEnergyCalculator.ModelTariffQuotes   
                                                  select q).Min();                     
}

This is my collection

    -       -       ModelEnergyCalculator   {EnergyHelpline.Model.ModelEnergyCalculator}    EnergyHelpline.Model.ModelEnergyCalculator
-       ModelTariffQuotes   Count = 4   System.Collections.Generic.List<EnergyHelpline.Model.ModelTariffQuote>
-       [0] {EnergyHelpline.Model.ModelTariffQuote} EnergyHelpline.Model.ModelTariffQuote
        ElectricityUsage    0   decimal
        FinalElectricityCost    179.97655091937073972602739726  decimal
        FinalGasCost    112.48534432460671232876712328  decimal
        GasUsage    0   decimal
        InitialElectricityCost  30.0117245403146958904109589    decimal
        InitialGasCost  18.757327837696684931506849312  decimal
+       QuoteIssuedDate {01/01/0001 00:00:00}   System.DateTime
        TariffName  null    string
        TotalCalculatedCost 341.23094762198883287671232875  decimal
+       [1] {EnergyHelpline.Model.ModelTariffQuote} EnergyHelpline.Model.ModelTariffQuote
+       [2] {EnergyHelpline.Model.ModelTariffQuote} EnergyHelpline.Model.ModelTariffQuote
+       [3] {EnergyHelpline.Model.ModelTariffQuote} EnergyHelpline.Model.ModelTariffQuote
+       Raw View        
+       ModelTariffs    Count = 4   System.Collections.Generic.List<EnergyHelpline.Model.ModelTariff>

How do I fix this error or is there a better way for getting the smallest value?

Answer 1

An easier and more flexible way of doing this than using IComparable is to order the objects as you need and take the first one. If you need the highest then you just need to use OrderByDescending.

var lowest = ModelEnergyCalculator.ModelTariffQuotes
    .OrderBy(m => m.GasFinalRate)
    .FirstOrDefault();

Answer 2

You should tell the compiler how to compare your objects by implementing the IComparable<T> interface for your class.Or if you want to smallest value of a property or field you can specity it in your query like this:

ModelEnergyCalculator.ModelTariffQuotes.Select(q => q.GasFinalRate).Min();

Or you can use overloaded version of Min:

ModelEnergyCalculator.ModelTariffQuotes.Min(q => q.GasFinalRate);

Answer 3

To use .Min() on a collection of a complex value you must implement the IComparable interface and specify how an object x is greater or equals to an object y. Otherwise there is no way for c# to know how to compare your values.

Alternatively you could use this yourCollection.Select(x => x.ThePropertyIWantToUseToCompare).Min() will return the smallest ThePropertyIWantToUseToCompare value from all the items in you collection.

Answer 4

The computer doesn't know how to compare a ModelTariffQuote with another ModelTariffQuote. So it can't find the minimum object. You have to define this comparison by implementing the IComparable interface on your ModelTariffQuote class. You can also implement the generic IComparable<T> interface.