example.cs

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using System;
using System.Linq;

var code = @"
using System;
#nullable enable

class C<T>
{
void M(C<int> p1, C<string?> p2, C<object?>? p3, object? p4) { }
}";

var syntaxTree = CSharpSyntaxTree.ParseText(code);
var compilation = CSharpCompilation.Create(
    "example.dll",
    new[] { syntaxTree },
    new[] { MetadataReference.CreateFromFile(@"C:\Windows\Microsoft.NET\Framework64\v4.0.30319\mscorlib.dll") });
var semanticModel = compilation.GetSemanticModel(syntaxTree);
var format = SymbolDisplayFormat
    .FullyQualifiedFormat
    .WithMiscellaneousOptions(SymbolDisplayMiscellaneousOptions.IncludeNullableReferenceTypeModifier);

foreach (var paramNode in syntaxTree.GetRoot().DescendantNodes().OfType<ParameterSyntax>())
{
    if (semanticModel.GetSymbolInfo(paramNode.Type).Symbol is { } symbol)
    {
        Console.WriteLine(symbol.ToDisplayString(format));
    }
}