Using LAMBDA functions

EPPlus supports Lambda functions from version 8.2. With Lambda functions you can define custom reusable formulas directly in the workbook, without needing VBA or external code.

Lambda in defined names

With EPPlus you can use Lambda functions with defined names, see the following example:

using var package = new ExcelPackage();
var sheet = package.Workbook.Worksheets.Add("Sheet1");
package.Workbook.Names.AddFormula("TestFunc", "LAMBDA(x,y, x/y)");
sheet.Cells["A1"].Formula = "TestFunc(12,3)";
sheet.Calculate();
var v = sheet.Cells["A1"].Value; // v is 4

Eta Reduced Lambda

From version 8.2, EPPlus supports Eta Reduced Lambda expressions. In Excel formula notation, this means you can pass a function directly as an argument to higher‑order functions like MAP, REDUCE, or SCAN, instead of wrapping it in a full LAMBDA. To make this work in EPPlus, you must prefix the function with the special _xleta attribute. This tells the calculation engine (and Excel) that the function is being used as a lambda reference rather than a direct call.

// Using full LAMBDA
sheet.Cells["B1"].Formula = "MAP(A1:A3, LAMBDA(x, UPPER(x)))";

// Using Eta Reduced Lambda
sheet.Cells["B2"].Formula = "MAP(A1:A3, _xleta.UPPER)";

Advanced Lambda usage in EPPlus

EPPlus’ calculation engine supports advanced scenarios with LAMBDA, enabling you to express logic that goes far beyond simple formulas. Two particularly powerful techniques are recursion and currying.

Recursion – a LAMBDA can call itself, allowing you to implement classic algorithms directly in Excel.

// Recursive factorial using LET + LAMBDA
ws.Names.AddFormula("Factorial",
    "LAMBDA(input, LET(n, input, IF(n = 0, 1, n * Sheet1!Factorial(n - 1))))");

ws.Cells["A1"].Formula = "Factorial(4)";
ws.Calculate();
// Result: 24

Currying – you can create functions that return other functions, building calculations step by step.

sheet.Cells["A1"].Formula = "LAMBDA(a, LAMBDA(b, LAMBDA(c, a + b + c)))(1)(2)(3)";
sheet.Calculate();
//Result: 6