C# – Where is the “Fold” LINQ Extension Method

cextension-methodslinqreduce

I found in MSDN's Linq samples a neat method called Fold() that I want to use. Their example:

double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 }; 
double product = 
     doubles.Fold((runningProduct, nextFactor) => runningProduct * nextFactor);

Unfortunately, I can't get this to compile, either in their example or in my own code, and I can't find anywhere else in MSDN (like Enumerable or Array extension methods) that mention this method. The error I get is a plain old "don't know anything about that" error:

error CS1061: 'System.Array' does not contain a definition for 'Fold' and no 
extension method 'Fold' accepting a first argument of type 'System.Array' could 
be found (are you missing a using directive or an assembly reference?)

I'm using other methods which I believe come from Linq (like Select() and Where()), and I'm "using System.Linq", so I think that's all OK.

Does this method really exist in C# 3.5, and if so, what am I doing wrong?

Best Answer

You will want to use the Aggregate extension method:

double product = doubles.Aggregate(1.0, (prod, next) => prod * next);

See MSDN for more information. It lets you specify a seed and then an expression to calculate successive values.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – LINQ query on a DataTable

You can't query against the DataTable's Rows collection, since DataRowCollection doesn't implement IEnumerable<T>. You need to use the AsEnumerable() extension for DataTable. Like so:

var results = from myRow in myDataTable.AsEnumerable()
where myRow.Field<int>("RowNo") == 1
select myRow;

And as @Keith says, you'll need to add a reference to System.Data.DataSetExtensions

AsEnumerable() returns IEnumerable<DataRow>. If you need to convert IEnumerable<DataRow> to a DataTable, use the CopyToDataTable() extension.

Below is query with Lambda Expression,

var result = myDataTable
    .AsEnumerable()
    .Where(myRow => myRow.Field<int>("RowNo") == 1);

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – LINQ query on a DataTable

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