[Java.Interop] JniTypeSignature & CultureInfo, empty strings (#1002)

Fixes: #335
Context: 920ea64
Commit 920ea64 optimized `JniTypeManager.AssertSimpleReference()` by
using `string.IndexOf(char)`.
This dovetails with Issue #335, which wanted to optimize
`JniTypeSignature` by removing calls to `string.Contains(string)`.
Fix#335, and update the `JniTypeSignature` constructor to use
`string.IndexOf(char)` instead of `string.Contains(string)`, and use
string indexers instead of `string.StartsWith()` and
`string.EndsWith()`.
`Java.Interop.dll` has no remaining usage of `string.StartsWith()`
and `string.EndsWith()`.
Additionally, update `JniTypeSignature` so that the empty string `""`
is *not* treated as a valid type signature. This is arguably a
breaking change, but the empty string never made sense, *and* would
throw an `IndexOutOfRangeException` with `JniTypeManager.Parse()`.

Author: jonpryor

src/Java.Interop/Java.Interop/JniTypeSignature.cs

@@ -43,11 +43,13 @@ public string Name {
 publicJniTypeSignature(string?simpleReference,intarrayRank=0,boolkeyword=false)
 {
 if(simpleReference!=null){
-if(simpleReference.IndexOf(".",StringComparison.Ordinal)>=0)
+if(simpleReference.Length<1)
+thrownewArgumentException("The empty string is not a valid JNI simple reference.",nameof(simpleReference));
+if(simpleReference.IndexOf('.')>=0)
 thrownewArgumentException("JNI type names do not contain '.', they use '/'. Are you sure you're using a JNI type name?",nameof(simpleReference));
-if(simpleReference.StartsWith("[",StringComparison.Ordinal))
+if(simpleReference[0]=='[')
 thrownewArgumentException("To specify an array, use the ArrayRank property.",nameof(simpleReference));
-if(simpleReference.StartsWith("L",StringComparison.Ordinal)&&simpleReference.EndsWith(";",StringComparison.Ordinal))
+if(simpleReference[0]=='L'&&simpleReference[simpleReference.Length-1]==';')
 thrownewArgumentException("JNI type references are not supported.",nameof(simpleReference));
 }
 
@@ -105,6 +107,8 @@ public static bool TryParse (string signature, [NotNullWhen (true)] out JniTypeS
 
 if(signature==null)
 returnnewArgumentNullException(nameof(signature));
+if(signature.Length<1)
+returnnewArgumentException("The empty string is not a valid JNI simple reference.",nameof(signature));
 
 inti=0;
 intr=0;

tests/Java.Interop-Tests/Java.Interop/JniTypeSignatureTest.cs

@@ -15,6 +15,14 @@ public void Constructor_Exceptions ()
 Assert.Throws<ArgumentException>(()=>newJniTypeSignature("java.lang.Object"));
 Assert.Throws<ArgumentException>(()=>newJniTypeSignature("[[I"));
 Assert.Throws<ArgumentException>(()=>newJniTypeSignature("Ljava/lang/Object;"));
+Assert.Throws<ArgumentException>(()=>newJniTypeSignature(""));
+}
+
+[Test]
+publicvoidDefaultConstructor()
+{
+vart=newJniTypeSignature();
+Assert.False(t.IsValid);
 }
 
 [Test]
@@ -55,6 +63,7 @@ public void QualifiedReference ()
 publicvoidParse()
 {
 Assert.Throws<ArgumentNullException>(()=>JniTypeSignature.Parse((string)null));
+Assert.Throws<ArgumentException>(()=>JniTypeSignature.Parse(""));
 Assert.Throws<ArgumentException>(()=>JniTypeSignature.Parse("java.lang.String"));
 Assert.Throws<ArgumentException>(()=>JniTypeSignature.Parse("Ljava/lang/String;I"));
 Assert.Throws<ArgumentException>(()=>JniTypeSignature.Parse("ILjava/lang/String;"));
@@ -78,6 +87,12 @@ static void AssertGetJniTypeInfoForJniTypeReference (string jniTypeReference, st
 Assert.AreEqual(jniTypeName,sig.SimpleReference,"JniTypeName for: "+jniTypeReference);
 Assert.AreEqual(arrayRank,sig.ArrayRank,"ArrayRank for: "+jniTypeReference);
 }
+
+[Test]
+publicvoidTryParse()
+{
+Assert.False(JniTypeSignature.TryParse("",outvar_));
+}
 }
 }