Add xunit tests.

Program.cs

@@ -1,11 +1,13 @@
-using System;
+using System;
 using System.Collections.Generic;
 using static System.Console;
+using Xunit;
 
 namespace Euler {
 
-  class Program {
+  public class Program {
 
+  	[Fact]
   	static long Problem1() {
   		long sum = 0;
   		for (long i = 1; i < 1000; i++) {
@@ -13,9 +15,11 @@ namespace Euler {
   				sum += i;
   			}
   		}
+  		Assert.Equal(233168, sum);
   		return sum;
   	}
 
+  	[Fact]
   	static long Problem2() {
   		long max = 4_000_000;
 
@@ -35,6 +39,7 @@ namespace Euler {
   			}
   		}
 
+  		Assert.Equal(4613732, sum);
   		return sum;
   	}
 
@@ -71,6 +76,7 @@ namespace Euler {
   		}
   	}
 
+    [Fact]
   	static long Problem3() {
   		long target = 600_851_475_143;
   		long targetSqrt = (long) Math.Sqrt(target);
@@ -81,6 +87,7 @@ namespace Euler {
   				highestPrimeFactor = i;
   			}
   		}
+  		Assert.Equal(6857, highestPrimeFactor);
   		return highestPrimeFactor;
   	}
 
@@ -94,6 +101,7 @@ namespace Euler {
   		return true;
   	}
 
+		[Fact]
   	static long Problem4() {
   		long largest = 0;
   		for (long i = 999; i >= 100; i--) {
@@ -107,9 +115,11 @@ namespace Euler {
   				}
   			}
   		}
+  		Assert.Equal(906609, largest);
   		return largest;
   	}
 
+		[Fact]
   	static long Problem5() {
   		for (long test = 20; ; test += 20) {
   			for (int i = 2; i <= 20; i++) {
@@ -117,12 +127,14 @@ namespace Euler {
   					break;
   				}
   				if (i == 20) {
+  					Assert.Equal(232792560, test);
   			    return test;
   			  }
   			}
   		}
   	}
 
+    [Fact]
     static long Problem6() {
     	long sum = 0;
       long sumSq = 0;
@@ -130,12 +142,17 @@ namespace Euler {
     		sum += i;
     		sumSq += i * i;
     	}
-    	return sum * sum - sumSq;
+    	long result = sum * sum - sumSq;
+    	Assert.Equal(25164150, result);
+    	return result;
   	}
 
+  	[Fact]
   	static long Problem7() {
   		List<long> primes = FirstNPrimes(10001);
-  		return primes[primes.Count - 1];
+  		long result = primes[primes.Count - 1];
+  		Assert.Equal(104743, result);
+  		return result;
   	}
 
     static void Main(string[] args) {

euler.csproj

@@ -3,6 +3,13 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>net5.0</TargetFramework>
+    <GenerateProgramFile>false</GenerateProgramFile>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.9.0" />
+    <PackageReference Include="xunit" Version="2.4.1" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
+  </ItemGroup>
+
 </Project>