如何在LINQ中对多个列进行分组

在SQL中与此类似：

SELECT * FROM <TableName> GROUP BY <Column1>,<Column2>

如何将其转换为LINQ：

QuantityBreakdown
(
    MaterialID int,
    ProductID int,
    Quantity float
)

INSERT INTO @QuantityBreakdown (MaterialID, ProductID, Quantity)
SELECT MaterialID, ProductID, SUM(Quantity)
FROM @Transactions
GROUP BY MaterialID, ProductID