C# Func<；T、 U，双>；作为构造函数中的参数

我从下面一段相对复杂的代码中摘录了以下内容

IDictionary<string, double> startingProbabilitiesDict = GetNewIDictionaryStringDouble();
IDictionary<string, double> transitionProbabilitiesDict = GetNewIDictionaryStringDouble();
IDictionary<string, double> emissionProbabilitiesDict = GetNewIDictionaryStringDouble();
...
var fbv = new SomeClass<string, string>
    (startingProbabilities: (s => startingProbabilitiesDict[s]), 
     transitionProbabilities: ((sOut, sIn) => transitionProbabilitiesDict[sOut][sIn]), 
     emissionProbabilities: ((s, o) => emissionProbabilitiesDict[s][o]));

我对这个符号感到困惑：

startingProbabilities: (s => startingProbabilitiesDict[s])

似乎认为

开始概率：

正在指定构造函数的第一个参数，但是为什么需要冒号，这个符号叫什么？

它被称为

名称是可选的，如果您将用法与可选参数结合使用，它会变得更有趣（下面的简化/哑示例）：

它被称为

名称是可选的，如果您将用法与可选参数结合使用，它会变得更有趣（下面的简化/哑示例）：

startingProbabilities: (s => startingProbabilitiesDict[s])

[Test]
public void NamedAndUnnamedTest()
{
    Assert.AreEqual("Only value1 was supplied", DummyMethod(value1: 1));
    Assert.AreEqual("Only value1 was supplied", DummyMethod(1));
    Assert.AreEqual("Only value2 was supplied", DummyMethod(value2: 1));
    Assert.AreEqual("Both arguments were supplied", DummyMethod(1, 2));
}

private string DummyMethod(int value1 = 0, int value2 = 0)
{
    if (value1 != 0 && value2 != 0)
        return "Both arguments were supplied";
    if (value1 == 0)
        return "Only value2 was supplied";
    return "Only value1 was supplied";
}